I believe it was Pokémon Diamond, and went through it for approximately two times in total. Next, it was Pearl. This, however, I did not get to finish; eventually, I wound up giving it to my friend, as she's never gotten chance to play any fourth-generation game at all. I'm glad I've given it to her, as it allowed us to battle with each other whenever we were free, trade, explore underground, and do lots of other fun things.
Later, I got Platinum, and it wound up to be my most favorite of fourth-gen, due to all its advanced features and whatnot. Pokémon Platinum--I've finished this game for about two times or so, like Diamond.