I'd say Greninja. It's the best one that has helped me a lot although I did rage quit the battle tree because of the rng. Here's the Greninja I used
Greninja @ Life Orb
Ability: Battle Bond
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Surf
- Ice Beam
- Dark Pulse
- Water Shuriken / Grass Knot