Water. Two weaknesses. Three resistances and two pseudo-resistances (Ground and Rock; it doesn't resist them, but kills 'em easily), nice variety, some very good typing (like Water-Ground), many with cool abilities, and even if the movepool is crappy, you can count on having Surf/Hydro Pump and Ice Beam, so if the Pokemon has good SA, you're good to go even if you have a crappy movepool =P.
You simply can't go wrong with a Water type. ^.^