If you want something defensive these Pokemon are all excellent walls:
Trevenant @ Sitrus Berry
Ability: Harvest
EVs: 252 HP / 4 Atk / 252 SpD
Careful Nature
- Will-O-Wisp
- Leech Seed
- Substitute
- Horn Leech
Chesnaught @ Leftovers
Ability: Bulletproof
EVs: 252 HP / 4 Atk / 252 Def
Impish Nature
- Drain Punch
- Leech Seed
- Spiky Shield
- Synthesis
Amoonguss @ Leftovers
Ability: Regenerator
EVs: 252 HP / 252 SpA / 4 SpD
Modest Nature
- Giga Drain
- Sludge Bomb
- Synthesis
- Spore
Tangrowth @ Leftovers
Ability: Regenerator
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
- Giga Drain
- Leech Seed
- Knock Off
- Toxic
If you want something with a bit more punch:
Shaymin @ Life Orb
Ability: Natural Cure
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Seed Flare
- Earth Power
- Hidden Power [Fire]
- Rest
Rotom-Mow @ Choice Scarf
Ability: Levitate
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Leaf Storm
- Volt Switch
- Hidden Power [Ice]
- Trick
