What's your least favorite game to replay of the old games? For me, it's DPPt. I enjoyed them when they came out, but they just don't have much replay value to me. The region is kind of boring and the game itself is just really, really slow, something I rediscovered while using the Poke Transporter on Pokemon Black recently.
I enjoy replaying Firered/Leafgreen and Emerald, but I have to say that Heartgold is more enjoyable to replay because of the updated gameplay and mechanics.