Water. The water benders have the best sub types of bending. Fire benders have lightning bending and combustion bending. Earth benders have sand bending and metal bending and lava bending. I don't think air benders have any sub types. Water benders have ice bending, spirit bending, blood bending, plant bending, and healing. Water bending has even been used to take people's bending away by using blood bending. Water bending is the best option.
Fire's spontaneous so always useful. That and Fire and Earth are really the only two factions with actual depth and moral grey areas. Air Nomads are morally perfect which irritates me to no end, and pre-Korra it was the same for the Water Tribe.