S:
Gyarados - Not a fan of using it because of how powerful it is, but you can't deny its strength. You can get an alpha Magikarp early enough, and it keeps up through the entire game.
Heracross - Yes, yes, we all know the exploits of Bois. You get it early, it starts powerful and remains so throughout the game. Flying moves destroy it, but it's not a particularly common type you see in this game.
Clawtizer - I'm surprised by how high I'm putting this up here, but Clawitzer is amazing. It's bulky, powerful, and has great coverage. Stack it with long-range moves and watch it snipe enemies from afar and tank hits for days. That said, I do use Alpha mons when I can, so the HP EVs probably contributed to its bulk. Still, I can only talk about my experience, and IME, it was my MVP in my 3rd playthrough.
Excadrill - Powerful and fast. Mega evolving, then Drill Running everything was my go-to in 2 of my games and was just sheer fun. Unfortunately, Fire and Ground coverage abound lategame, and can take it out if you're not careful.
Garchomp - Same as with Gyarados, I don't like OP mons, but you have to respect its power. Evolves a bit too late, though.
A:
Clefable - Surprisingly bulky with self-sustain in Draining Kiss and a wide movepool. As long as you're not fighting anything with Poison or Steel STAB, you can bring it out and have it just slowly chip them away.
Florges - Same as Clefable. It has bulk, self-sustain, including Giga Drain, and dual screens.
Aromatisse - Again, ditto Clefable. Bulk, self-sustain, dual screens.
Skarmory - Late mon, but it starts putting in work from the moment you get it. Its typing is great defensively, and its Mega allows it to function as a mini-Mega Excadrill.
Altaria - Bulky and has a great typing. Ice moves aren't common, so Altaria has longevity in most fights and can just slowly chip its way to victory.
Medicham - Mega Medicham is phenomenal. It's powerful, has great coverage, and melee windups are fast. Unfortunately, it evolves pretty late, it's frail, and it requires its Mega form, but its power more than makes up for these downsides.
B:
Meganium - Bulky, can self-sustain with Giga Drain, and has dual screens to reduce damage further. Unfortunately, it has a lot of common weaknesses, and its middling speed and long cooldown moves (especially if you have a support move equipped) mean that it doesn't attack as much as you would like.
Glaceon - Surprisingly good. It's powerful, Ice is a great offensive type, and it doesn't mind its lack of coverage too much since you can get lucky and lockdown enemies in constant Frozen states.
Krookodile - Strong and has great STABs, but is also a bit too frail. It often ended up in a 'kill or be killed' situation for me.
Aegislash - The unsheathing animation of the entire line kills it, but it's particularly egregious with Aegislash. It can fire off powerful moves, and Steel is a great typing, but I didn't really enjoy my time using it.
Frosslass/Chandelure - Similar mons, IME. Chandelure is more powerful, and Frosslass has lockdown potential, but their Ghost typing leaves them particularly susceptible to Crunch, which is very common late game.
Camerupt - A weird mon. It's not the most powerful, it falls to any Water move, and Earthquake is commonplace lategame, but once it gets up close to an enemy, Lava Plume and Bulldoze/Earthquake can make short work of them without the long windup of long-range moves.
Simisage/Simisear/Simipour - Fantastic early - mid game mons. Evolution stones are easy to come by, so you can have powerhouse mons from ~ level 14 to the mid 30s, especially since all 3 get 100+ BP moves at level 32. Sadly, they fall off hard around Rank D/C due to their frailty and lack of coverage.
Dragonite - Very good once you get it, but unfortunately, it comes too late at level 55. Til then, you're stuck with Dragonair, which is just mediocre, but is saved by how good Dragon is defensively.
Eelektross - Another weird mon like Camerupt. There were times when it felt powerful and bulky, but there were also times when it was weaker and frailer than I expected. I'm really not sure where to put it.
Aggron - Very similar to Medicham in that it needs its Mega to function well. It's too frail without it, but unlike Medicham, it doesn't get that much more powerful with it. Solid mon, but there are better.
Arbok - Great early - mid game mon. Can learn all the elemental fangs, and their low cooldown means that you can cycle through them quickly. It falls off once Earthquake becomes more common.
Vanilluxe - Similar to Glaceon, but evolves late and is less powerful. Still, Ice is always good, and locking down enemies happens more than you'd expect.
Heliolisk - Surprisingly not too bad. You get it a bit too late, and it's physically frail, but Thunder and Surf make it take down more mons than you'd expect.
Glalie - Ditto Vanilluxe, just weaker.
Malamar - A good mon when you need something with neutral typing. As long as the enemy doesn't have Bug or Fairy moves, you can bring Malamar out and have it chip its way to victory.
C:
Dragalge - Man. One of my most favorite mons finally got a Mega, with a sick design, but it evolves late, isn't too powerful, weak to Earthquake, and has middling physical bulk. It falls easily and doesn't take many things out. Shame.
Scolipede - I love its Mega design, but in the 2 games I've used it, it just doesn't do much of anything. It's not particularly strong or bulky, unless it's in its Mega form, but even then, Fire and Rock coverage are ubiquitous lategame, not to mention that Bug and Poison aren't really great offensive types. Alpha Scolipede is also pretty big, and long-range attacks can fly over targets' heads if you're not careful with your positioning.
Vaporeon - I expected it to be bulkier than it was, but most story enemies dogwalked it every time I brought it out. It also has terrible coverage.
D:
Aerodactyl - Was Aerodactyl always this frail? It was dying so easily to everything, and its damage wasn't anything groundbreaking to justify the frailty.
Aurorus - I love this mon down, but its typing leaves it susceptible to many attacks. I thought it'd fare better in this battle system, but no, it's as bad as usual.
