From experience, it depends on the purpose.
My vote goes to: F-air, Nair, Bair and D-Air
Fair is because it has enough Knockback for certain gimps and can be used for Combo's if used correctly.
Nair is because it can stop juggles if the opponent is trying to assault you on the air and also combo to the ground and lead to a rapid jab infinite (Although it's difficult) (This is also besides the other cool **** it can do)
B-air because it can actually Gimp opponents if you use it to gimp with the stage if they don't tech it. It carries a big risk since you can be gimped yourself if the opponent knows what they are doing.
D-Air is because not only is it the only move that jab locks and leads to good punishes, but is also, against certain recoveries, his only efficient Air Gimp. (Ex. Falco, Fox, DK, etc.)
As for U-Air. It's pretty useless. The only time I ever see it used effectively is as a quick poking tool to earn some extra damage in very specific scenarios.
Those are my thoughts. Hope it was useful