Bulbasaur probably has the best typing for a grass starter. 2 of its many weaknesses are eliminated and resisting fighting is always nice. Poison immunity is great too, especially when facing those Weedle in gen 1.
I've always wanted to try a Regenerator Amoongus. Sounds fun. Hope it's easy to get/train one in XY.
Roselia gets a special mention for being one of the few gen 3 Pokémon I like.