Snivy cuz I'm basic and I love Serperior
second place would be Chikorita, followed by Turtwig and Rowlet who lose points because I hate their evolutions. :[
My favorite Grass-type starter is Rowlet since I love birds, especially owls, and it's quite cute.
Grass is probably my favorite starter type of the three generally. I don't dislike any of them, though I am kind of disappointed that Meganium is so mediocre. The Bulbasaur line stands out for being the original and for having a nice Mega Evolution. The Snivy line stands out for me as well since it was my starter in Black & White which, while I had been a fan of the series since the beginning, Black & White were the first games where I could trade and battle with people online so they hold a special place in my heart. Also really happy that Serperior has Contrary to work with these days.
Snivy because snakes are awesome and Serperior is so regal looking. A lot of people had problems with it losing legs as it evolves but I don't really have a problem with it. Meganium is also pretty good.
I love Treeko to the point of soft resetting to get a shiny Treeko. Next would probably be Rowlet or Turtwig. I?m honestly suprised over how many people like Snivy the best. From what I can tell, he/she is universally unliked.