Water, because-
-You can easily regain HP, like with Water Veil, or with the ability Water Absorb.
-You can power up, or speed up your party in the rain.
-You can prevent heavy damage from Self-destruct, or Explosion with the ability Damp.
-You can get in some pretty vicious attacks in like Hydro Pump, or Hydro Cannon, or even Surf.
-And.... the water pokemon look really cool!
[some of the things i listed here do not pertain to all water pokemon, except for the looking really cool part, i was just generalizing what you could possibly do with water pokemon by specifying attacks and abilitys, to help you get an idea of how awesome water pokemon are]