I chose Turtwig in Diamond, and I'll do the same in Platinum.
Why? I'll tell you:
1. If I chose Chimchar, I'd have to get a Roserade. Too generic.
2. Torterra is wicked.
3. Personally, I do not like Empoleon. Great typing, great sp. attack, horrible speed to go with it.
Actually, I got Platinum a week ago, and when I got Turtwig, I transferred over a Chimchar egg so I could have them both.
You have been Claytonated.