For me, it's a tie between water and electric types. There are so many good water types out there and surf is not only one of the most used HM's in the games, but it is (in my opinion at least) the best water move in the game. A lot of good electric types can you find early on in the game and i'm still suprised how much pokémon like Ampharos, Luxray and Zebstrika can handle. I've beaten Red in SS today for the first time (never got around to it, did it because a friend wanted a charmander for his birthday) and Ampharos took down almost every pokémon of Red's team. Even though I have some very good pokémon in my team (Typhlosion, Tauros, Hypno, Lapras and Dragonite) but Ampharos is my lead. and that's why I've ultimately voted for Electric type's