Waterbenders can heal. That means you can recover after most fights. This is a HUGE advantage. Waterbenders can draw water from many sources, like water, plants, moisture in the air, sweat, saliva etc. They even carry a water skin with them. Thus, waterbenders does not rely on 100% on the external source of water. For combat, they can turn their opponent's force on themselves.
Powers - max 100/100
Offensive power - 50 + opponent's power/100
Defensive Power - 80/100
Earthbenders have their powers balanced. That means they have equal offensive and defensive powers. Earthbenders totally depend on the earth and minerals like crystal, metal etc. Once they are not in contact with earth, like when Toph is on Appa, and on icy paths in The Serpent's Pass as their powers are completely negated as they have nothing to bend at all.
Powers - max 100/100
Offensive power - 75/100
Defensive Power - 75/100
Firebenders only focus on their offensive powers. Throughout the series, Firebenders have shown numerous types of attacks. But, they are really weak on defensive powers as they can only use Fire Wall as their shield. They seldom use them though, as firebenders rely on their agility like Azula, Zuko and Ozai to evade most attacks.
Powers - max 100/100
Offensive power - 80/100
Defensive Power - 10/100
Airbenders, the complete opposite of firebenders (although stated in the series that the opposite is Earthbenders) in powers, has magnificent defensive powers. Airbenders can create air barriers, wind shields etc. to block most attacks. They can also evade most attacks by their super speed, with or without the Air Scooter. However, they lack fatal moves. They don't have deadly strikes, only normal ones. This is maybe because they are harmonious and never started war. They view life as sacred and rarely kills. Thus, they only concentrate in defensive powers to defend themselves.
Powers - max 100/100
Offensive power - 10/100
Defensive Power - 80/100