While HG/SS were really well done remakes, my pick goes to D/P/Pt. While Diamond and Pearl did have a few problems with their releases, they brought along some revolutionary changes, most important being the Physical/Special split, something that's helped to reshape the competitive metagame. The story was a breath of fresh air, and among the more creepy and dark stories in the Pokemon series. I liked Platinum especially because it was a Pokemon main series game that didn't seem so much like Pokemon at all. Cyrus' plans, the Distortion World and Giratina itself made the game darker than a Pokemon game should be. Platinum introduced a lovely cast of characters, like Looker and Charon, while HG/SS reworked the story of Gold and Silver and made good use of current-generation technology to create a crisp, clean game and something that looked brand-new. I liked Sinnoh more partially due to nostalgia, and the fact that I loved the locations in Sinnoh and the story Diamond, Pearl and most importantly, Platinum gave me. No hate to HG/SS though, they were excellent also.
Oh, did I really forget to mention Platinum's my favourite Pokemon game?