The best Grass type in White is probably Petilil. This is because it's caught early, has good matchups into Clay and common Water types, and the potent combination of Own Tempo + Petal Dance. Though Lilligant is fairly one-dimensional, lacks coverage, and struggles 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.