Porygon2
85/80/90/105/95/60 BST: 515
Porygon2 is the only Stage 2 Pokemon with over 500 BST. It also has an unique Normal typing for a Stage 2 Pokemon making it a neutral choice for most Stage 3 cross evos as it has one weakness, no resistances, and one immunity. Porygon2 also has a nice movepool meaning it can benefit from some "setup centric" movepools (i.e. movepools with setup moves). Porygon2's natural BST also means it can have a hight BST total depending on its evolution making it a nice choice for bulky special attacker cross evos such as Hydreigon.
Magmar
65/95/57/100/85/93 BST: 495
Magmar is another one of those amazing Stage 2s because of their standard evolution being introduced in Gen 4, meaning Magmar was meant to be a final evolution. Magmar has a really good sized movepool, meaning it can appreciate potentially gaining a type and, with its decently mixed offenses, it appreciates a plethora of potential evolutions as well.
Electabuzz
65/83/57/95/85/105 BST: 490
In a similar vein to Magmar, Electabuzz is a really versatile Stage 2 thanks to its mixed offenses and great natural Speed stat. Electabuzz also has a lot of coverage moves such as Focus Blast, Psychic, Low Kick, and Fire Punch meaning if it can cross evolve with a Pokemon that has access to boosting moves, it will be a very big threat to watch out for.
Rhydon
105/130/120/45/45/40 BST: 485
Arguably one of the best Stage 2 Pokemon out there, Rhydon offers immense physical bulk and attacking power. It appreciates gaining special bulk from its new cross-evos while a new ability is better than anything it has thanks to it not having Solid Rock like its original evolution. Ground and Rock are also really solid typings, meaning it can easily pair with other typings to make a really nice offensive or defensive combo.
Magneton
50/60/95/120/70/70 BST: 465
Another one of the greats, Magneton has awesome special stats as it has a better initial Speed stat from its standard evolution as well as roughly the same power. The typing it has is really good as well although it does come with some common weaknesses. Magneton is probably best off cross evolving with Pokemon such as Hydreigon to gain an average Speed boost as well as a massive amount of Special Attack.
Dusclops
40/70/130/60/130/25 BST: 455
Dusclops brings amazing defenses and a great defensive typing, making it a top notch defensive Foundation. Its decent movepool means it already has access to moves such as Will-O-Wisp, Trick, Calm Mind, and Shadow Sneak. Give it a cross-evolution that provides reliable recovery and a decent secondary typing and Dusclops would make for a nice Extreme Speed immunity as well as an amazing wall.
Golbat
75/80/70/65/75/90 BST: 455
Golbat has a pretty decent movepool itself with access to things such as Brave Bird, Roost, Taunt, Defog, Nasty Plot, and Toxic. It also has a really nice, somewhat even, stat spread that can allow it to become a really good special attacker or powerful physical attacker with a decent amount of speed. Its typing, should it remained unchanged, is pretty decent as well with a 4x resist to Fighting- and Bug-types.
Sliggoo
68/75/53/83/113/60 BST: 452
Sliggoo sports an amazing special movepool with moves such as Thunderbolt, Ice Beam, and Sludge Wave. Sliggoo also has amazing special bulk as well meaning with the right Foundation, it can be a great special wall. Sadly, Sliggoo lacks recovery in its own movepool, meaning a Foundation with it is nearly a necessity.
Piloswine
100/100/80/60/60/50 BST: 450
Piloswine has amazing STAB coverage with access to STAB priority as well making it a really effective Stage 2 Foundation. Giving this a cross-evo with access to Swords Dance or Dragon Dance with an Attack or Speed boost makes this a very potent Foundation to watch out for.
Doublade
59/110/150/45/49/35 BST: 448
Doublade has an astounding Defense stat and an excellent typing, making it one of the few Stage 2s that would rather have a Masterclass or Specialist cross-evo that
doesn't change their typing. Doublade also appreciates being able to gain a new ability as No Guard isn't beneficial to most of the moves it currently has, which is has nearly everything in its own movepool for a nice bulky physical sweeper with Swords Dance, Iron Head, Shadow Claw, Shadow Sneak, and Sacred Sword.
Klang
60/80/95/70/85/50 BST: 440
Klang is a really nice option thanks to it having access to the unique Shift Gear, which is basically a better Dragon Dance, and the fact that it appreciates being able to gain new moves and possibly a new STAB from cross evolving. This means things such as Salamence and Dragonite are very nice options as it gives it Skarmory's typing while also providing it with a massive amount of moves (sadly no good Flying STAB).
Lairon
60/90/140/50/50/40 BST: 430
Lairon makes for a really solid Foundation as it has a decent Attack stat and an above average 140 Defense stat. It also appreciates a Masterclass or Specialist that can change its typing as Steel offers up defensive capabilities making it a decently good wall while Rock offers up good offensive coverage which can turn it into a powerful attacker. Sadly, it does lose out on a recoilless Head Smash, which may deter one from using it as a Foundation.
Metang
60/75/100/55/80/50 BST: 420
Metang is a really decent Foundation boasting STAB priority in Bullet Punch as well as a plethora of coverage and potential STAB moves such as Grass Knot, Earthquake, and Hammer Arm. Sadly, it has a low Speed stat, but with the Steel typing, it can become a fearsome wall / Fairy killer while being able to get around a low-ish Speed stat thanks to priority.
Gurdurr
85/105/85/40/50/40 BST: 405
Gurdurr has a really nice mono Fighting typing with STAB Mach Punch and Drain Punch. It also has access to Bulk Up, meaning a decent Masterclass or Specialist that buffs its HP, Special Defense, and Speed can make it a very viable bulky win condition.
Haunter
45/50/45/115/55/95 BST: 405
Haunter has two amazing type should it cross with an evolution that changes it type, making it very versatile since it has access to quite a few moves already. It sits in a similar boat as Kadabra as it benefits from the Specialists such as Hydreigon.
Togetic
55/40/85/80/105/40 BST: 405
Togetic may not offer up much in the ways of BST, but it does offer a decent movepool and an excellent defensive typing, meaning it can be a very versatile Foundation. Having a low HP stat means its bulk is a tad comprised so it would really appreciate having that beefed up as well.
Kadabra
40/35/30/120/70/105 BST: 400
While Kadabra is very frail, it has a very high Special Attack and Speed stat meaning it can benefit from evolutions that offer up high boosts in these such as Hydreigon. Kadabra does hate losing its old abilities, but this can make it a very big offensive threat to watch out.