Platinum for its extra features, Battle Frontier and tweaked plot. Not to mention, the new outfits were pretty cool compared to the ones in Diamond and Pearl (not to say those were bad). While I'm at it, some of the newer sprites were a lot better than those in the first two versions; Heatran's sprite for example, that was hot stuff.
Before I looked in this thread, I was sure everyone would say Platinum.
...at least a majority did anyway. XD