there are too many to choose from.

FF1. plain airship so its nothing fancy. definetly not the best.
FF2. same thing.
FF3. ive not played it.
FF4. the airship has a lot of upgrades but nothing to make it better thant the rest. the whale i really good. it has a fat chocobo and a free inn and can travel anywhere but the underworld (which is sort of annoying because you have to find the other airship to get down to the underworld.) the whale is possibly the best.

FF5: this one is one of the best too. its an airship,ship and submarine convertable. most of it makes no sense. sometimes you see the airship deck but when you go to the submarine mode you see a completely different place. the submarine has a resting place so its definetly a good airship. but its not 100% an airship so i dont think it really counts.

FF6. nothing special but the backstory behind the airship and the names and their plain coolness makes me like them but there not as useful as other airships.

FF7. highwind. meh. nothing fancy. nice music though and the pilot has some funny moments E.G. when hes a level 14 pilot and diamond weapon has risen from the sea, you talk to him and next to his name it says "limit break" in flashy letters. the highwinds good and really fast.

FF8. ragnorok. its a spaceship and ive never really found it that special. looks cool and has big cannons and guns though.

FF9. hilda garde 3 and the invincible. i like the invincible.

FFX. cids ship.

FFX-2. the celsius. has a barkeeper and places to rest but the music and looks really annoy me and i try not to stay in it for too long.

i suppose the most useful airship is the whale or the FF5 ship (if you count it as an airship.)

there are so many and i really cant decide.