Alright, the Water Benders are the best if they are near water, and especially if there is a full moon. AND waterbenders can heal
Air Benders are extremely fast and evasive, if they can wear their opponent down they can move in for the kill.
Fire Benders are good because of it's raw strength, but as Jeong Jeong said, it can't really be used for helping others without hurting someone, or making a fire.
Earth Benders are strong and tough, and useful.

I don't know if anybody can really say what the strongest element is. We'd be here forever argueing if that were the case. It is largely a matter of taste.

If you like quick moves that strike fast, immobilize your enemy, and overwhelm them you would say Water.

If you prefer strong, sturdy attacks that hold immense power and can manipulate the ground underneath one's feet (most of the time) you would choose Earth. If you like flashy moves that look great, do a fair amount of damage, and have a chance at scarring your opponent, you'd say Fire.

If you want to tire out your enemy--strafing and dodging their attacks while waiting for a weak spot to show--and gain the ability to fly (and flying gives you the chance for sneak attacks and more evasive maneuvers) you'd choose Air.

They have their weaknesses and all, but they're balanced out for the most part.