Platinum for me, too. The way I view Pokemon games is every successive title is better than the last, so 2nd gen was better than 1st gen (within reason), 3rd gen was better than 2nd, and 4th is the best thus far. Yes, it's debatable, but I'm looking at the games as a whole - and I certainly did find 3rd gen an improvement over 2nd.
But yeah. Platinum takes from all the other previous games in the series and adds a bit more. Now we can rechallenge the Gym Leaders and an upgraded E4 in a single game, we have tons of new move tutors, lots of new ways to play the game via direct connection and the WFC, new forms for various Pokemon, and the Battle Frontier's back. Add to that all the things we already had from D/P (Morning/Day/Night, the attack-type split, and the Pal Park) and Platinum is easily the best one yet. I even find the new plot and layout for the gym leaders fairly nice, so it's not just Platinum's features that make it great. The music for the Distortion World and the Giratina battle are pretty damn awesome, too.
But yeah, like Carmen said, all of the regions have their own charm, so I can't call Platinum the end all, be all Pokemon game. Some people may just like Kanto/Johto/Hoenn more as a place, and that's cool.