Piplup on my very first playthrough of both games.
I've used them all, but this time round in Diamond, I chose Chimchar due to lack of good fire/fighting-types, then got a Piplup egg because I couldn't find a decent natured Buizel and traded mine away for a Chatot in-game.
On Platinum, I tend to be swayed towards Turtwig, mainly as Magmortar, Gallade, Houndoom among others are available due to the expansion