Turtwig is my favourite. I love bulky Pokémon, and I enjoy its movepool and design, too. Honestly, I'd keep it from evolving if I were crazy; Turtwig is sooooooo cute~! But Torterra is a BEAST.
I've picked Piplup more often, though, just because it seems to fit better on the teams I plan to use. In Platinum, I like using a Leafeon with Yawn, so...
I've never picked Chimchar. Monkeys used to be my favourite animals when I was a child, but I hate the way Chimy's entire line looks, so much so that I won't use it... Shame, 'cause I know it's a great Pokémon.
By the by, has anyone ever noticed the possible Gen I influence on these Starters? Venusaur has a giant flower growing out of its back; Torterra has a giant tree growing out of its back. Monferno has the Charmander line's fiery tale. And Empoleon is of the Steel-type, possibly due to Blastoise and its steel cannons...?
;)