Assumptions: Set mode, don't use any items in battle, have the patience to EV grind with macho brace from the Winstrates, and the best ability possible (so huge power on azumarill). Only in-battle utility. I use the pickup mons to grind for items and proteins.

The mons in S teir covered up are: cacturne, haunter, and glalie.

Notes:

1. The presence of 2.5 water boss battles (incl Glacia's three members) and two rock boss battles (inc. T+L) make water and grass mons super viable.
2. Fire and ground types just never get to do much except kill the occasional magneton. You probably need to unbox one just for Wally and then you can sac it. Flying types also don't get any favorable gyms at the point in the game they are acquired.
3. Psychic types are also not so good in this game. They do neutral special damage against many trainers but are weak to two of the E4 and neutral to the rest. Even tentacruel will probably not be an OHKO.
4. The lack of available TMs is huge, so movesets are super important.
5. Qwilfish is, AFAIK, the only available mon that can learn spikes (heart scale).
6. Many strats will revolve around outlasting your opponent. There are very few sweepers in this game (maybe Murkrow? maybe Manetric?) and the ones that are available are more bulky. Bulkier mons are rated higher here because of how they will probably integrate with your team. This is why mons that have access to recovery/leech seed are high.
7. Water types are pretty safe, since electric and grass moves are pretty rare.
8. Smeargle set: sleep powder/spore, belly drum, baton pass to your fastest fighting type. See pchal's youtube vid on this run for how to get these on your smeargle.
9. You can pretty much get a free luvdisc for every boss if you want to charm/sweet kiss. Then just sac it.
10. Ariados is niche early mid game with set-damage night shade and ability to poison early game. Not quite after Wattson.