I'll share a few thoughts on some Pokemon I used in my playthrough:
Vivillion: A solid early game mon and an incredible sweeper midgame once it gets quiver dance, compound eyes-boosted sleep powder and bug buzz (and later, compound eyes-boosted hurricane). Since it's a bug that evolves incredibly early, it's quite strong early game with a 410 BST at level 12 (you might think it gets destroyed by the fire gym, but I found it to be the second best pokemon on my team after rockruff in the fight.) It falls off somewhat after, until it gets all the tools it requires to sweep, which given the weirdish level pacing of desolation, happens pretty quickly. It is somewhat sleep turns dependent on sweeping, but with a small amount of luck, it has one of the highest highs of any sweeper. I'd put it in A.
Panpour/Simipour: The two things simipour has going for it is that it (in the form of panpour), learns scald at lvl 22 and acrobatics at lvl 31 (although both are only as panpour, having to delay evolution to get acrobatics is definitely annoying), which are two good moves that are really strong at that point in the game. (It's also one of the best pokemon for thief+recycle+fling cheesing, but I'm not even sure if that's in Desolation). I'd put it in D.
Alolan-Muk: An incredibly strong typeline, solid stats, and a very strong level up movepool. The primary downside is that it comes pretty late in the game (I believe it is in the Cellia sewers, although my memory is a little hazy). Probably belongs in B.
Decidueye: One of the better starters, with a very good level up moveset, with access to good priority and U-turn (although U-turn is definitely a lot worse than it is in competitive). It gets nasty plot by level up, which is also a really good enabler for a special or mixed set, but isn't nearly as good as it is in other games since there is no access to shadow ball yet, so you're stuck with omnious wind for special ghost STAB. Downsides are that it gets kind of destroyed by the first gym, and it's hidden abilities aren't that great. I see it as a B.