The best Grass type Pokémon is probably Roserade. It has a nice 125 SpA stat and a nice 90 base Speed stat. While it doesn't learn any good moves outside of Petal Dance and Giga Drain by level up, it does have access to a nice TM movepool of Sludge Bomb, Shadow Ball, Dazzling Gleam and Energy Ball. Roserade also needs a Shiny Stone to evolve, but that is available on Route 12, so not too far into the game. Budew is found on Route 4 as well.