The best Grass type in White is probably Petilil. It's caught early, has good matchups into Clay and common Water types, and benefits from the potent combination of Own Tempo + Petal Dance. Though Lilligant is fairly one-dimensional, lacking coverage and struggling against Trainers like Burgh, these traits are not enough to detract from the sheer strength of its Grass STAB.
Lilligant
Ability: Own Tempo
- Quiver Dance (Level 28 as Lilligant)
- Giga Drain (Level 26 as Petilil)
- Sleep Powder (Level 10 as Petilil)
- Petal Dance (Level 46 as Lilligant)
You will want to evolve Petilil at either Level 26 or Level 27 so it can learn both Quiver Dance and Giga Drain.