I'm unsure if these are allowed, they seem cliche, but whatever, I'm new, so I wouldn't know. I like Chimchar mainly for Infernape and the fact that I love monkeys. I like Piplup mainly for Prinplup and the fact that I love penguins. And I like Turtwig for Grotle and the fact that in Diamond and/or Pearl, if you choose Turtwig, the rival chooses Chimchar, and Dawn chooses Piplup, so it's a balance.
Me: Dusk
Rival: Vincent
Rowan's Assistant: Dawn
Turtwig: Turtwig (Dusk) , Chimchar (Vincent) , Piplup (Dawn)
Piplup: Piplup (Dusk) , Turtwig (Vincent) , Piplup (Dawn)
Chimchar: Chimchar (Dusk) , Piplup (Vincent) , Piplup (Dawn)
Plus, choosing Turtwig follows the story. The hero has Turtwig (Ash), Dawn has a Piplup, and the rival has a Chimchar (Paul).
So I'd say Turtwig.