Like almost everyone else, I have to go with Platinum. It's the upgraded version of both Diamond and Pearl, and since Diamond and Pearl are both the same thing with only a few pokémon difference, Platinum wins. I like to battle a lot and when I heard they had the Battle Frontier I declared it to be my favorite game out of the three straight away.
Also, I see no reason to dislike Platinum in any way because there's just a few added features and different pokémon levels. Otherwise, everything else is almost quite the same. However everyone has their own opinion, and this is just mine which shouldn't mean no more to anyone else. xD;