Generation 2 brought in some great ones. I had some difficulty deciding between it and Gen IV, with Gen V coming in as a close but distinctly less influential 3rd. Gen IV split moves into special and physical and introduced some great Pokemon, but Gen II introduced the Steel and Dark types, entry hazards, breeding, weather, held items (in addition to trading with held items), and amazing new moves. It brought Suicune, Umbreon, Shuckle, Blissey, Tyranitar, Scizor, Kingdra, Skarmory, Smeargle, Celebi, Crobat, Politoed, Raikou, Espeon, Heracross, Sneasel (which was great even before there was Weavile), and even Steelix, Entei, and Forretress. It was definitely the most influential generation of them all.