Gen 3 and Gen 5. Gen 3 because they were the first games that I really had an idea of what I was doing (and because Hoenn and Colosseum/XD were great IMO), and Gen 5 because there have been so many great features introduced. I also enjoy Platinum (pretty much the only game from Gen 4 I really like), and Yellow will always be special to me since it was my first Pokemon game.