In my Diamond, I started with Piplup, so I'll probably start with either Chimchar or Turtwig, this time. However, given that there is a greater number of Fire types, this time around, (since the Platinum Sinnoh Dex has 210, instead of 150) I will likely go with Turtwig. Granted, I actually have the three Starters on my Diamond, so it wouldn't really matter, either way. XD
But for a change in pace, I might go for Turtwig, since I'd rather have the versitility in Water and Fire types.