Tinted Lens
Stamina
Filter
I think that Tinted Lens gives this CAP an offensive edge that is really nice because it can hit almost anything consistently well, something that most Pokemon can't do. It's a very unique advantage that, if paired with a set-up move, could be incredibly viable. This would be valuable for offensive and defensive Pokemon alike - pretty much any Pokemon stands to benefit from dealing more damage to the enemies that resist their attacks.
Stamina is just a favourite of mine - I've used it effectively with Mudbray in my Little Cup experience, and it can be really strong if used well. But if used badly, or if the opponent doesn't play into it, it can be overcome with ease. It's an interesting dynamic, and one that can be fun to explore along with moves such as Stored Power, Body Press, Counter or Bide. If this CAP is going to have a defensive forme, this would be fun to see.
Filter is good for reducing damage from super-effectives, so it's a nice pick for a Pokemon with one or two troublesome weaknesses. Other than that, I'd say it isn't as fun to build strategies around as the prior two. One thing I'll add is that this CAP could be themed around weaknesses and resistances, with one forme taking Tinted Lens, and the other taking this ability.