Air-theres aways a supply....unless you were in a vacuumm,(which isn't in the avatar world....so they dont have to worry about that ) Not the best for offensive attacks...but great defence. They can use their bending to run faster and make their attacks quicker also. The nation itself is more on the neutral side though.

Water-great supply. Convers almost 3/4 of the earths surface. Not always in supply though when on land....unless you are near plants and stuff... Water is great with both defence and offence. A Waterbender "lets their defence become their offence, turning their opponent's energy against them." They can also bend the ice and snow. And they even have bloodbending...even if it is for only 1 night every month.

Earth-also great supply. Although there is less on the actual surface, there is always some earth buried deep underwater. Also has great deffence and offense. Earthbenders need to be able to touch the earth to bend though, so flying and being in the middle of the ocean is a disadvantage. The people themselves are strong like their element, so they could probably fight well even without bending.

Fire-since they create the fire...there is always going to be a supply. If they are cold however, they cant bend very well, if not at all. Fire is very offensive, but still has some defence. Because fire is very destructive, it normally wields the most damage. Also, some select few firebenders can also bend lightning.

There really is no "better" element. All the elements have their advantages and disadvantages. It only depends on which one you like the most

I myself would prefer water because it has the most bonuses.