It's gotta be Pokémon Emerald for me; it had a lot of features that the others didn't have, such as animated sprites, Battle Frontier, etc. It's also the only game I had ever tried completing my Pokédex in, haha. Was pretty close, but couldn't get it finished successfully, though, unfortunately. I was lacking Feebas and Bagon, and regardless of how much effort I put in, I just couldn't get a hold of them. :( Besides Emerald, I also really like FireRed/LeafGreen.