I had to say Generation I and Generation III. Gen I, IMO, had the best Pokémon, and the most creativity. Gen III had a lot of new features, and was the first generation I played, as well as having Colosseum and XD: Gale of Darkness in it. Generation IV seemed like they were getting a little desperate for designs, and just started adding new evolutions, while trying to make it appeal more to people above the age of six. Gen V holds some promise, as all the Pokémon (I think) seem to be from scratch, with no evolutions from previous generations. Gen II was great, but really didn't throw itself at me as much as I thought it could have, there were great features, and many great Pokémon (Misdreavus is one of my favorite Pokémon), but it seemed to be missing something to me.