If by Grass type you mean the starters, then you know what my answer is: FAGperior.
Because giving insulting nicknames to pokemon is ever-so-mature.
Mine would have to be Chikorita. I don't hate it, but it's my least favourite.
No offense, but Serperior is a design I hate so much, that I have a hard time calling by its name, and I end up referring to him by an insult. Many people have characters they hate so much that they do this.