Out of the ones I've played, Platinum. That's right :P
Got a problem?
I won't be able to play HG/SS until later this month (even though it comes out...today!). But when I do play HG, that will top the list, most likely.
I just liked Platinum because in my opinion, all of the extra features made it better then the classics, (including Crystal). I can see why some fans prefer the same old Pokemon, but I have to disagree that not adding new Pokemon would be better. Sure the newer designs are not on par with the simplicity of the old designs, but new Pokemon are what make each game special, in my opinion. And personally, I hope they finally achieve having 700-1000 Pokemon. That way, whatever 6 Pokemon you use will be a reflection of you. I prefer it so that no two teams are alike (unless you enter the metagame, where EVERYONE uses the same ubers/OUs Pokemon.)
Sorry for my annoying chatter, but I will continue:
Platinum was also my favorite because of Wi-Fi. HG will probably be my favorite, and I can't wait for my Chikorita to follow me around.