Turtwig, Chimchar, Chikorita, and Cyndaquil.
First time I played through Diamond, I chose Turtwig. That didn't quite work out and I restarted after finding it difficult to beat the Elite Four, so I chose Chimchar. Chimchar became what I chose the most after that point, and Infernape is my favorite final starter evo of all the starts. In Platinum, I chose Chimchar. Right now I restarted my Platinum, and Turtwig is my starter this time around.
First time I played through HGSS, I did Cyndaquil. Second time I played through HGSS, I did Chikorita. Third time I played through HGSS, I went back to Cyndaquil.