Although Gen 1 will always be the one I hold closest to my heart (my entire childhood revolved around Pokemon Red & Yellow), I think Gen 2 fixed a lot of the issues with Gen 1. Firstly, the introduction of Dark and Steel types REALLY helped in balancing out the game. Then they introduced held items, many more attacking moves, the ability to Surf without opening the menu (god I hated that in gen 1), and it also had the longest single player mode, even though I think the Kanto portion was a bit rushed. All in all Gen 1 is the greatest because it introduced Pokemon, but Gen 2 really fixed a lot of the issues with it.