Dang. Oh well, if Will-o-Wisp hits Slaking, it's still a lot less worse than it was before.
Think I'll add another few!
Xatu, which is available as Natu in Amethyst Cave, for B(-)/C(+) rank!
Xatu has a few nice moves, including Tailwind, Air Slash, Confuse Ray, Wish, Psychic, and Ominous Wind, all pretty early. Tailwind and Wish are nice support moves, Confuse Ray is pretty good to have, Air Slash and Psychic are two of the best STABs it gets, and Ominous Wind can just be there for the chance to boosts stats. Also, Teleport for outside battle (although Teleport and Dig were broken in Reborn, so I don't know how they'll be in Rejuvenation).
Some battles:
Venam - N/A
Keta - Xatu is very nice if you overlevel it for Air Slash and Psychic and then lower its levels. It hits everything on his team really hard, basically.
Other gyms - While I haven't used Xatu in these battles, Wish is nice support for all of them. Also, Xatu might be able to do quite a bit of damage in Crawli's battle. As for Narcissa and Angie? I wouldn't use this one.
Camerupt, available as Numel in Caratos Mountain, for B(+) rank.
Camerupt has plenty of great things about it. It has a great movepool with amazing stats to back it up. Although it has quite a few weaknesses and a lackluster speed, this thing is amazing.
Some moves it gets include: Amnesia, Curse, Lava Plume, Eruption, Earth Power, Rock Slide, Yawn
I'd recommend this thing for pretty much every Gym besides Valerie for obvious reasons.