Pokemon Tier List
Note: The best items are the gold boosting teeth, the x-items, everything that boosts crit and the -mana+resistance items. The worst items are the ones that gives a bunch of +3s and the red/blue orbs which are far too expensive.
S Tier - Free Win
Mawile - Starts with a broken signature move that gives more stats than anything else in the game. Avoid stat boosting items and go all in on economy, health, resistance and crits. You can get 100% crit change by floor 5 or so. Then you just outdamage and kill everything.
Cacnea - Can get not one, not two, but three drain moves. Need I say more? Same as above. Unkillable because you have multiple moves that heal almost your full HP bar as well as strong attacks.
A Tier - Good.
Marill - Cacnea but with lacking damage. My first run I got good items and won. My second run, I didn't see a single good damage boosting berry or item and died after 20 turns against Torterra. Basically a tutorial pokemon.
Sableye - Starts with Shadow Ball and Power-Up Punch. Learns Shadow Ball (again) and Shadow Claw. SPE is such a terrible stat there's never a reason to use it. ATK and SPATK.
Sneasel - Another crit pokemon. 100% easily. Kill everything with Shadow Claw. You can one or two turn bosses easily.
Scyther - Sneasel but has buff moves instead of Shadow Claw. Worse over all but Fury Cutter is great. Much slower fights but still consistent. You really want a Shell Bell though.
Croagunk - Ignore poison, get crit and strong physical moves. Win.
B Tier - Solid.
Taillow - One of the rare SPATK pokemon. The roost double team combo isn't as good as Marill or Cacnea but is great combined with strong attacking moves. Get a shell bell and you'll win 100% of the time.
Riolu - Surprisingly, not many great special moves. I was waiting for them to show up and they never did. Just run physical attacks and it's an okay pokemon.
C Tier - Bad
Pikachu - The most confusing pokemon imo. I had no clue what to build into. I still don't. Just pick the best items you get and if you end up using all three stats, that's okay. Ignore the charge gimmick because it's useless. By the end of my run, I'd get 10 charges every two turns or so, which compared to the 15 energy per turn I get was useless.
Cubone - A better debuff than poison, but still terrible. Ignore the debuff and get physical moves. Lower ranked than Croagunk because while the debuff is better, available attacks are worse.
F Tier - Shuckle tier.
Shuckle - Starts with three garbage moves and Wrap. Curl upgraded is okay. Its best ATK move is Headbutt but Shell Smash gives headbutt +2 damage. Resistance is a terrible stat. I got an early x-attack and that wasn't enough. The only pokemon I couldn't win with.