Game: Tie between R/S/E (I've played all three), Pearl, SoulSilver, and XD: Gale of Darkness.
Haven't played much of Generations I and II; I found Black to be a really good game but it was lacking in some departments; I think X is kind of a mess, but still a worthwhile experience. Also, my brother bought Alpha Sapphire, and he offered to let me play it, but I haven't gotten around to it yet.
Pokemon: Flygon.