LeafGreen was the first one I went back to - picked it up on the same trip that saw me nab a brand new copy of Platinum not long after it came out (a big regret is that I didn't know it was coming out, so restarted my long-running Diamond save for the car journey and thus lost a bunch of Pokémon from 2005 - now my oldest is only 2011). Given I'd seen original series anime, it was fun to finally get to use all the originals (Gen I 'dragons' - Charizard, Gyarados, Dragonite - have long been my favourites). It still holds up, the simplicity and having actual choices of direction in those and HgSs are refreshing compared to the relative prescriptiveness of the Gen III, IV, and V main games.
I think I picked up Black 2 a couple of years after release - the quick turnaround of HgSs-BW-B2W2 caught me out - but I've only completed it once as it was pre-owned and kept crashing around the time the big ship appears.
Oh, and I had Battle Revolution for Wii a couple years after it came out, which was great fun to see proper 3D battles, although because it was only battling I did tire of it quickly. My parents were very reluctant to get me games though, forcing me to take awful trade-in deals when I wanted a new game, so it and about 6 others collectively got me the latest edition of FIFA that year. Atrocious short-term decision-making, especially for independent business owners!
I doubt I'll ever buy a new Pokémon game now, but I would definitely consider getting XD if it didn't also mean having to track down a GameCube, Shadow Lugia looked so cool!