I liked them all for different reasons, but gen 5 has had by far the least playtime of all sevens gens with me; I just couldn't get into it.
The best games, though, were Gold/Silver/Crystal, and their remakes, HeartGold/Soulsilver. Gold and Silver just added so much to the originals, with breeding, the day/night cycle, held items and more, plus they had the best postgame: the entire Kanto reigon with another 8 gym badges to get! Then Crystal added the option to play as a girl, which was the best thing ever, being a girl myself. And Heartgold/Soulsilver just made these great games even better!