I voted Gen II. The reason being is that it introduced several aspects that would remain staples to the generations that followed, including Pokemon genders, breeding, equippable items, happiness, day/night, the Battle Tower, Dark and Steel-types, roaming Pokemon, shiny Pokemon, different formes (Unown being the first real case), and so on.
Gen III, though controversial, was also important due to the things it introduced on a competitive standpoint. Still, Generation II gave me the biggest initial "wow" factor that no Gen has given me thus far.
Gen IV was also a great generation, but it merely added a little to the competitive aspect in terms of mechanics, most notably the Physical/Special split. Other than that, its mechanics were merely a Gen 3.5. There was also HeartGold and SoulSilver, which made us relive the glory days of the metal generation, but that's all it did. It wasn't like we haven't seen it before.
I've yet to touch Gen V, but from what I hear from reviews, it may just make me like Gen V over Gen II. Until the games are actually released, however, I will say that Generation II has to be my favorite of the first four that I have played because of the impact it had in terms of innovation to the games.