Absol -
A rank
My greatest reservations with Absol in A are that it comes rather late, is underleved, and is somewhat frail. After thinking it over, however, I think access to SD + Night Slash should be enough to hold it over as it catches up to the rest of the team. Its mega evolution has high mixed offenses and speed (plus Magic Bounce protects it from status and debuffs), though its access to coverage moves is somewhat limited by gameplay elements. Bonus points for faring well in gym 7 and the E4, however.
Electrike -
A -> B rank
I don't think Electrike is A material, tbh. While it does get an earlier Spark, it loses access to special Dark attacks (which would help against Wattson) and is forced to rely on its lower offensive stat until Charge Beam and Volt Switch. Remember that it is also rather frail and has few noteworthy resistances after Wattson. Charge Beam is admittedly a decent move but I find that it is still rather weak and isn't guaranteed the SpA boost; Volt Switch makes you switch out. It fares better against Team Aqua than Magma but isn't impressive at all against Flannery or Norman and can't mega evolve until after the Lati sidequest. At that point, it starts to pick up, boasting SE hits on Winona and the sea routes of Hoenn as well as much stronger power with Intimidate as well. Overheat + Hidden Power (despite being rather weak) helps to round out its coverage. If I had to sum it up, you either catch and use Electrike as soon as you can but then struggle for a few badges before getting good (ie B material) or you wait until you can Surf, catch it at a level closer to evolution but still much lower than your other mons, and use it when it has actual advantages (still B material).
Groudon -
A rank
Groudon is easily one of the best pokemon available in Omega Ruby. Behemoth physical stats and good bulk / speed as regular Groudon which are augmented to titanic levels in its primal form, it boasts great offensive typing and a killer ability as well as an expansive movepool. Additionally, it comes with at least 3 perfect IVs as a member of the sterile breeding group. I can't think of anything that can truly stand up to it for the remainder of the game. In my opinion, its ideal movepool is PBlades / Lave Plume / Solarbeam / Dragon Claw. Such a set allows it to sweep the 8th gym, victory road, and the E4 in one fell swoop; the only downside is that Solarbeam requires you to have made a detour through the Safari Zone earlier in the game and Dragon Claw requires you trekking all the way back to Meteor Falls.
Heracross -
A rank
Despite being a rather late addition to the team, Heracross is clear A-rank material. Good dual STABs, bulk, sky-high attack (especially as mega), and acceptable speed make it a good choice. It is caught already knowing Brick Break and gets Pin Missile 1-3 levels later, with Megahorn shortly thereafter. Heracronite is actually available before Tate and Lize, but using it in this gym may be risky as it is weak to Psychic despite its Bug-type advantage. Still, it hits extremely hard in all of the following matches and is capable of pulling its weight. The E4 falls to it easily (Phoebe can be managed with Shadow Claw if you caught a Guts one) and it is helpful against Steven to an extent. Its downfalls include late acquisition and below-average speed but I think its positive traits make up for it.
Kyogre -
A rank
See Groudon, pretty much. If anything, Kyogre is even better as its STAB is boosted by permanent rain and its ideal moveset hardly requires any detours (just buy Thunder at Lilycove). While Groudon is immune to the 8th gym, Kyogre simply overpowers the other water-types and its greatest fear is Sweet Kiss confusion and the like, which is not likely to be an issue. It fares just as well as Groudon against the E4 and champion given its monstrous offense and decent speed. Its lateness is the only reason it isn't S.
Magnemite -
A rank
I was also borderline on this one but decided I was being a bit harsh in the end. Magnemite has a tough early game. It is obtained via Horde using Sweet Scent (Sturdy or the rare analytic would be best) at Lv 6, meaning that it has significant catching up to do. Fortunately, Sonicboom helps a bit. Magnemite is of average performance against major bosses and fights until Norman, but it performs much better in the field thanks to coveted Steel typing and the good neutral coverage of the electric-type. If you can get Magneton by the time you fight Norman (you should), its high defense helps there, too. After that, mid-late game is where it really shines. it is a similar story to Manectric, only you trade speed for bulk with substantially more power and a secondary typing without needing to mega evolve. Magnezone can be obtained via a quick detour to New Mauville once Wally warps you back to Mauville; a moveset of Flash Cannon / Volt Switch / Tri Attack / Thunder Wave could theoretically carry you until the end of the game, but you do get Discharge and Hidden Power as other options. It's low speed is somewhat of a let down but honestly it isn't that important considering it resists 12 types.
Makuhita -
A rank
Makuhita is pretty consistent throughout the game. It's a hard hitter with slightly above average bulk and low speed. If obtained via Slakoth trade (it should be, DexNav makes Slakoth easy to find), it gets accelerated experience points and is useable for Roxanne's gym. It performs well in the early-mid game gyms and then becomes better for field trainers later on in the game, but by then other team members can help pick up the slack. Fighting / Dark is essentially perfect coverage as the fairy type is pretty rare in this game, which leaves Hariyama with two open slots for Strength or whatever you need. Unfortunately, it is outsped by many things and this leaves it prone to status and frequent healing (though Guts remedies this somewhat). Team magma / aqua can be annoying as they have Zubat and Golbat that can frustrate Hariyama; it regains some use late-game against the E4 and champion.
Marill -
A rank
Like XY, Marill has excellent typing for progression through ORAS, and its revamped level up movepool is even more conducive to this. Though I wouldn't recommend its use against Wattson, it does very well against Flannery and has the bulk to take on Norman and the rest of the gyms. STAB Huge Power Aqua Tail / Play Rough by level 25 is ungodly; throw in Facade or Strength to complete the coverage as nothing in the game resists it. Azumarill is also a naturally good user of Dive and Waterfall if necessary. Like Hariyama, Azumarill's biggest downfall is its low speed, leaving it prone to status and chip damage. It also has a weakness to poison, which can come into play against the Golbat / Weezing / Muk of Team Magma / Aqua. Otherwise, it has a good matchup against most trainers both in and out of boss battles and deserves its A rank.
Mawile -
A -> B rank
I don't think Mawile is A rank material. For starters, it requires backtracking to Granite Cave to get it once you've beaten Wattson and comes at a whopping Lv 12, requiring serious babying. It fares well against most field trainers but dislikes the various ground- and fire-types (Numel) found on the way to Lavaridge. It does get access to early Iron Head and Play Rough by Fallarbor (significant as it requires 2 heart scales that other pokemon may need, instead of the usual 1) but is not useful in Flannery's gym with its low speed and weakness to fire. It does pick up against Norman and beyond, especially with Mega evolution, but it is still quite slow. I feel like Mawile is in an awkward place as you either have to backtrack and baby it but still have to wait a while for the payoff, or catch it in the cave of origin where it has missed out on its peak usefulness. That said, it does have a really good matchup against the E4 with Fairy / Dark / Steel coverage.
Meditite -
A -> B rank
Meditite also belongs in B rank, I think. It comes around the same time as Absol, which I gave A rank, but I think there are a few key differences. For one, Meditite is extremely frail with piddly 30/55/55 defenses. It relies on HJK for all of its damage output which is strong but risky, and it is still 5 levels away from evolution. Once it evolves into Medicham, it will need a Heart Scale for Zen Headbutt and possibly one of the punches depending on your needs. Mega Cham is sufficiently fast and strong that nothing can stand in its way. However, Medicham does not have any gym advantages and can find itself outsped by a variety of pokemon, many of them being Team magma / aqua's Golbat given the teams greater presence late-game. Medicham still remains as frail as ever, though to be fair it will knock out most pokemon in one hit. It fares well against half the E4 and is alright against the others, and does decently against Steven as well. If I had to sum it up, Medicham is a great pokemon that would benefit from coming a bit earlier in the game and maybe being a little faster. I think B is fine for it.
Camerupt -
A rank
This thing is great. It is pretty much the only viable Fire-type in the game outside of Torchic. Fortunately, Numel comes relatively early in the game, has immediate power thanks to Magnitude and Flame Burst (upgraded to Lava Plum and Earth Power by level 26) and good offensive typing. As Camerupt, it gains Rock Slide and average bulk with 70 / 70 / 75 defenses and 100+ offenses. This pokemon performs best early-mid game as late Hoenn has alot of water; OR players will find a short period of relief during Groudon's Desolate Land, which can actually be used for training via Solarbeam and a water immunity. Camerupt has a neutral matchup against many of the gym leaders and E4 but its high-powered STABs and good offensive stats keep it relevant throughout the game.
Ralts -
A -> S rank
Ralts is one of those pokemon that has pretty much always been controversial. The root of the issue is this: does the time spent as Ralts / Kirlia justify the wait until Garde? I think the answer is yes, for ORAS at least. Unlike XY, Ralts and Kirlia have useable Fairy STAB early on in the game in the form of Disarming Voice AND a gym where they can excel thanks to a 4x resist and SE offensive coverage. Early Hoenn has a bunch of Poochyena for Ralts while mid Hoenn sees alot more poison, though they don't actually have great poison coverage (Gulpin / Zubat / Roselia) - this works out in Ralts and Kirlia's favor. I think this lessens the earlier forms' dependence on babying in the early game, especially since Magical Leaf helps with mid-game water- and ground-types. Kirlia even has Lv 23 Draining Kiss to offer some longevity and it pairs well with Calm Mind at 26. Gardevoir is even more beastly in this game than in Kalos. Psychic comes at Lv 30 as Kirlia and Moonblast is just a heart scale away, giving Garde some of the strongest power and coverage in the game up to and even beyond that point. By Lilycove, it gains the Focus Blast and Shadow Ball TMs for appropriate coverage. Finally, Gardevoir gets another power spike just after the climate crisis resolves, as its mega stone becomes available at Wally's house. Mega Garde can sweep the E4 with a set of Moonblast / Shadow Ball / Focus Blast / filler.
Shroomish -
A rank
A tier is fine for Shroomish. It excels against the first gym and Mega Drain can hold it over until it becomes Breloom around Wattson's gym; Mach Punch / Mega Drain / Rock Tomb / filler covers it from there. You can be hopeful and get lucky with Seed Bomb or Bullet seed early on even though I know we aren't counting on that. Breloom has high power and decent bulk / speed, allowing it to sweep Wattson and even come out on top against Flannery's Slugma / Numel as it outruns them. It fares decently against Norman due to being able to take a hit for the 2HKO with Mach Punch and another fighting move. It fares a bit worse against the remaining gyms and struggles a bit against team Aqua / Magma, but you can't have it all. The E4 is not a worthwhile endeavor outside of Sydney and maybe Glacia. Solid A rank.
Treecko -
A -> S rank
Another controversial one. I feel like Treecko is similar to Electrike and Magnemite in that while they can be obtained early, they tend to shine more late-game. The main difference is that Treecko requires no extra effort to obtain or train up; it is also the best grass-type pokemon in the game. Treecko's Absorb is adequate for the very early game since you're mainly seeing Wurmple and Shroomish as is and Mega Drain comes in time for Roxanne. Rock Tomb is immediately teachable for flying- and bug-type pokemon. It can usually 2HKO Brawly's pokemon through Mega Drain -> overgrow Mega Drain but its a risky strategy and best to use another pokemon like Taillow or Ralts for him. Treecko should have Giga Drain by the time it gets to Wattson and has access to Low Sweep via TM, so he can handle this gym although not efficiently as Marshtomp or Combusken.
Grovyle is fast and strong enough to OHKO Zubats with Rock Tomb and Giga Drain will for sure KO Carvanha. Numel can be a bit tricky. Mine could stomach no more than one Wing Attack / Air Cutter from Golbat but Rock Tomb is a 2HKO so there is some use there. Grovyle fares alright in Flannery's gym thanks to Dig being 80BP now, and it can take some of the non Fire-types like Meditite and Kecleon, but it should not be relied on against Flannery as Overheat, Sunny Day, and Light Screen make its life hard. I haven't tried it, but I imagine that if you somehow managed to SD on Slugma you could potentially Dig your way to victory.
Grovyle is too frail for Norman but can manage in his gym as living a hit + overgrow Giga Drain usually kills. Once mega, Sceptile can clean house for the rest of the game. Using a heart scale for Leaf Storm is doable but not really necessary as it can do all it needs to with SD / Dual Chop / Dig / Giga Drain. You can use Rock Tomb as necessary for Winona's gym but STAB Dual Chop is as effective and much better against Altaria. Sceptile sweeps the sea routes and remaining two gyms thanks to its high speed and natural advantage, which has the added benefit of keeping it healthy as well. It has a neutral matchup against most of the E4 (avoid Glacia) but can beat Drake by using SD before mega-evolving and then using Dual Chop on the turn it Mega evolves.
Wingull -
A -> B rank
I like Wingull. It has the perfect typing for early and even mid-game somewhat, but I think its stats let it down as the game goes on. Roxanne is a gamble as Geodude will survive with Sturdy and Wingull doesn't like the speed drop / damage from Rock Tomb; it'll normally come out on top though. Early Wing Attack keeps it useful against the early grass- and bug-types and gives it an edge in Brawly's gym, but Wattson is a no-go for sure. It does well against Flannery thanks to Water Pulse and is a great choice for Norman thanks to base 100 defense and Protect shutting down the Slakings. After this point, though, it tapers off. Base 85 SpA is adequate to use with Surf but its best flying STAB is inaccurate and it otherwise has base 50 attack. It isn't particularly powerful and is also slow and not super bulky, which leaves it wanting in later battles despite access to Roost. It is far too weak to be of use against the E4. Generally speaking, I think Pelipper is great early game but the fact that it just falls off after the 5th gym lets it down; it is also totally outclassed by Gyarados with the exception of Dewford island when you still have Magikarp. As an example:
Lv 28 Gyarados 16 IV / 0 EVs Thrash vs. Lv 28 20 IV / 0 EVs Vigoroth: (40.91% - 48.86%)
Lv 28 Pelipper 16 IV / 0 EVs Water Pulse vs. Lv 28 20 IV / 0 EVs Vigoroth: (30.68% - 37.50%)
Even despite having STAB Water Pulse, Pelipper is outdamaged. It doesn't start to win until Surf comes along to even things up for Winona but by then, Gyarados should have Aqua Tail (and Ice Fang makes it moot anyway).