I would like to ask why Aron is considered D tier Pokemon. I was thinking about using it along with Trapinch in my Emerald playthrough but I'm not sure. It has adequate attack, amazing defence and usable early STAB options (Metal Claw and Rock Tomb) while having many useful resistances (albeit possessing horrible weaknesses as well). This is a serious question, not a complaint about tiering.
Aron's main issues can be summed up as a typing that fails to shine in most major battles and evolutions that frankly just come a bit late for their payoff. I actually use Aron more often than I probably should because I think Aggron is a cool pokemon, so I'll go through a mixture of my experiences and what Aron should/should not be able to do beyond that. Aron's best point is honestly right around when you catch it, since it's coming basically with Headbutt and Metal Claw or at least incredibly close to them, and has the option to take the only slightly contested Rock Tomb TM you just got. At this point you have a pokemon with more than respectable attack for that point in the game, an interesting defensive typing backed up by a sky high defense stat, and access to both STABs (granted at 50 BP) and a relatively high power coverage move for that point in Headbutt.
This is immediately unable to be put into any real use due to Brawly being a fighting type gym leader and absolutely annihilating Aron. The next major fight afterwards with the rival has both Marshtomp and Combusken as a complete wash due to their secondary STAB, although Grovyle isn't overly difficult for Aron to beat, though still somewhat annoying. In Ruby and Sapphire the Wailmer is also very difficult if not almost impossible, and both the Shroomish and the Numel are both unpleasant for Aron to try and deal with. In Emerald the Wingull instead of Wailmer is much easier to deal with due to Rock Tomb being much more effective, and both the Slugma and Lombre (since Lombre's best move is Absorb instead of Shroomish's Mega Drain) are significantly more plausible to deal with.
Wattson is a chore for Aron. While Aron does handily stop Voltorb's Selfdestruct from being a threat, the electric STAB attacks every member of his team has hurt a lot due to Aron's lackluster Special Defense and HP. The Magneton (and Magnemite in RS) are arguably the easiest to deal with due to the Mud Slap Aron has - absolutely horrific vs every other opponent but finally getting some potential use - but even then it's still a weak non-STAB move and Aron's attack is no longer as stellar as it was just a little while before.
Maxie/Archie at Mt Chimney is the next major battle in your way and at this point Aron is getting old and you just want to get to evolution please so Aron can stop being so dead weightish. Mt Chimney does set the tone for basically all of Magma and Aqua vs Aron though, basically being that Aron does amazingly well vs all Poochyena and Zubat lines and is basically a dead stop for most of Team Aqua in general including Archie since Archie's best shot is right there with Sharpedo's Crunch. The next battle has him relying on Swagger and Slash (woo hoo). Maxie and Team Magma meanwhile have Poochyena and Zubat that are just as ineffective but the Numel lines that so many of them just completely ruin Aron and evolution's day since virtually all of them have Magnitude or Earthquake. Overall Numel is hilariously good vs the team that likes water for basically the entire game, but struggles vs Team Magma whenever they use Numel is an accurate summary of Aron vs evil teams.
Flannery is the next major battle and it doesn't go quite as well as you might hope. One of Aron's flaws that I haven't gone into huge detail about is that it's really slow, so it's often taking hits before it gets a chance to retaliate, burning through healing items more than faster pokemon do. This shows up somewhat in Flannery's gym, though many of Flannery's own Pokemon are slow so you might be able to take them out before they nail Aron with high powered STAB attacks on its poor special defense. Aron definitely does better in RS than Emerald though.
Norman is arguably the major battle where Aron finally gets a chance to shine, and remember earlier when I said that right after getting it was Aron's high point? I lied, this is Aron's high point, right here and in this battle. Without Norman getting incredibly lucky Aron should be able to take Norman without much issue, purely due to its resistances. You've also probably just gotten to Iron Tail at this point, and who better to try it on than Norman? Just don't use it on the second Slaking in RS, nothing worse than missing it when Slaking goes for Focus Punch. If you've ground past Norman with Aron up to 32 and evolution to Lairon things are even easier due to Lairon's flat out superior stat spread.
Before Winona ideally Aron's evolved into Lairon which has helped patch up its flagging attack stat back to "good" levels. Aron's weaknesses and strengths are just highlighted with this evolution, although its weaknesses are slightly patched up. Lairon's special bulk is still lacking, its speed is still terrible and you get hit first more often than not still, but your defense continues to be sky high and your attack, backed up by your shiny new high BP move Iron Tail, is pretty good.
Route 119 Rival and Aron depends almost entirely on whether or not you've managed to get to evolution, which is a higher level than this rival battle's ace. If you have not evolved, Ruby and Sapphire rival is a complete wash, don't even bother really since the only thing you have a shot against is the Shroomish, even Grovyle is probably 1v1ing Aron. As Lairon this changes slightly, since you can probably take on Grovyle at that point and almost certainly beat Shroomish. Numel, Wailmer, Combusken, and Marshtomp are implausible to beat. In Emerald the prospects are much better, since Lombre is terrible depending on which move Nature Power turns into (I believe it's something that doesn't change the matchup right there), Pelipper is only running Wing Attack, and Slugma doesn't have Magnitude like Numel does. Aron can almost definitely beat Pelipper, Slugma, and most likely Lombre depending on what Nature Power is, and still really needs to be Lairon to take on Grovyle.
Winona seems like an idea fight at fight from Lairon's typing alone - especially if you gave Lairon Rock Tomb and honestly why wouldn't you? However, a significant portion of Winona's team is pretty difficult for Lairon, specifically the Pelipper and Altaria. Pelipper is beatable, although it'll do a very large amount of damage to Lairon first. Altaria is a dead cause - it has Earthquake in all versions which Lairon cannot handle. Skarmory and Tropius are both plausible to beat, but they're also very annoying to try and take out, particularly Skarmory. If you still have Aron at this point, which is completely possible since level 32 before Winona is something you could not hit, especially with something as annoying to grind as the slow Aron, you can almost definitely beat the Swablu or the Swellow, but the Tropius and Skarmory are a less likely cause without items. Pelipper also multiplies in difficulty.
Next is Lilycove Rival, who's almost identical to the Route 119 Rival only you should definitely have a Lairon by this point. Matchups are basically the same, although there's now a Swellow you eat for breakfast and Emerald Rival insists on proving they're a moron by Combusken in Emerald having Quick Attack instead of Double Kick, so now Lairon eats it for breakfast too.
After this your next major battles are Team Aqua/Magma who I summarized overall matchup earlier, and then we finally get to Mossdeep. At this point Lairon is very difficult to use vs average trainers as well, since you're on the water and Lairon is a rock type and slow. RS Tate and Liza are pretty simple for Lairon, both being weak to steel and the most threatening move being Solrock's Flamethrower which doesn't hit all too hard unless Solrock gets sun up. Emerald is much more difficult since Lairon cannot afford to be out until Claydol dies since Earthquake is still Earthquake.
After this there's more Aqua/Magma as summarized much earlier, and then finally you're at Sootopolis gym for your next major battle. Odds are you're not Aggron right now, since 42 is somewhat tricky to actually arrive at since your main opposition up to this point has been Hoenn's oceans. You might have arrived there by feeding off Team Magma's Poochyena and Zubat and Team Aqua in general, but I wouldn't put money on it. Regardless, this would only change your matchup in a small way, since now you probably beat the Luvdisc. Other than that it's a water type gym and they all actually have a STAB move, what did you expect?
Aggron continues its pre evolution trend of not taking much from anything except fighting and ground physical attacks, hitting actually pretty hard at this point, being slow as hell, and having mediocre at best special defense. As Aggron you get access to a pretty cool move in Brick Break which is actually pretty contested and Aggron probably doesn't really need it even though it'd like it, and you've only recently gotten Earthquake which is just the most heavily contested physical TM you'll get so good luck Aggron. Regardless, giving Aggron these moves turns it into a slow but powerful physical attacker with several strong options, although Rock Tomb is still your Rock STAB. You also get a bunch of special TMs but Aggron's special attack is absolutely terrible so don't bother - they're worth nothing on Aggron outside the "cool" factor.
At this point we're at the League for major battles. Wally you do fairly well against except Magneton if you don't have Brick Break or Earthquake, but if you're bringing Aggron this far you honestly should. Aggron probably won't take on all of Wally's team, they do wear him down with (resisted) special attacks since Aggron is slow, but he can take on at least one, probably two or three without items.
Elite Four I'm going to sum up very briefly.
Sidney: Does pretty well especially since you're probably a higher level if you're fighting league, Cacturne is annoying and Sharpedo/Crawdaunt are inadvisable to fight.
Phoebe: Will-o-Wisp from Banette sucks and Dusclops 2 has Earthquake but otherwise it's just a slugfest that you win due to type and power alone.
Glacia: In RS both Sealeo and Walrein have Water STAB as well as their Ice moves. Avoid fighting them. In Emerald this is only the case for Walrein, so you can take on the Sealeo. Glalie is no challenge and is weak to your Steel STAB.
Drake: Weird bastard, you should definitely beat Shelgon and Altaria, in RS one of his Flygon has Dig and the other Sand Attack so you can stay out for at least one turn vs either of them, Salamence is annoying due to Intimidate and a fairly strong Flamethrower, Emerald Flygon has Earthquake and Kingdra has surf so don't fight them.
The Champion is obviously very different between RS and Emerald, but neither of them are really favorable to Aggron. Steven first, where his Metagross and Claydol are both washes and unless you feel confident your Aggron is faster and you've given it Earthquake so is Steven's Aggron due to all three having their own Earthquake. Armaldo and Cradily aren't too hard to take down as long as your iron tail doesn't miss over and over, but Skarmory is a prolonged battle that you definitely win if you use items and probably win even if you don't, but he's also going to wear you down and get spikes up. I'd advise somebody else for Skarmory there.
I would be incredibly surprised if an Aggron at comparable levels managed to take out more than one of Wallace's pokemon, and I'd even be caught slightly off guard if Aggron managed to 1v1 any of them. Wallace is a bad matchup for Aggron.
These have been overall a mixture of my experiences with Aron and looking in more depth at the movepools of major battles, and I hope this more clearly explains why I believe Aron is much more a D rank pokemon than any other rank. Sorry for the absolutely massive post, I actually quite like Aron and so had a lot to say about it.