Tropius for HM slave, Roserade for battling.
Tropius has that horrible Grass Flying combo (x4 Ice weakness), with no saving grace like Shaymin-S has with Serene Grace & good speed/ sp. atk. Roserade, however, has a very nice Grass/ Poison typing, meaning Bug, Ground and Poison are only x1. It's also got a good speed & sp. atk, along with decent sp. def, but low def & HP so watch out for that.
Tropius @ (no item because it's a HM slave)
Ability: (who cares?)
Tropius learns Rock Smash as well, but imo it's probably better to tack that onto a fighting type for some STAB.
Roserade @ Black Sludge (if you can get it)
Ability: Natural Cure
- Giga Drain
- Sludge Bomb
- Energy Ball
First three moves for STAB, last move to replenish health & switch out to wake to up. You can choose Leech Seed or Toxic Spikes over Energy Ball if you want.