I actually went for the Turtwig line in terms of being useful. Despite having quite a few weaknesses, including a four times weakness to Ice type moves, I thought it was the most useful out of the Grass types I've raised with winning quite a few battles for me in my Platinum team. Though, I didn't make the best use of either my Treecko or my Bulbasuar when I raised them in their respective generations, so they're probably stronger competitively. In terms of my favorite, the Treecko line easily wins. I loved the design of all three of its form, with Treecko being the cutest of the Grass type Pokemon in my opinion, and it does have a pretty nice movepool. Bulbasaur is pretty good, even though I always thought that Ivysaur looked the best out of all three forms due to looking so tough, but my Venasaur in Blue and LeafGreen was pretty good. The Snivy line is pretty good design wise, but I was rather disappointed with how Serperior turned out while raising one in White. It just wasn't nearly as strong and useful as I had hoped it would be. The Chikorita line would probably be my least favorite since, while all forms are kind of cute, they weren't that great when I raised one in my second copy of Gold and the movepool doesn't do much for me.