In terms of aesthetics:

Best: The Blackjack, I love the Zeppelin design of the ship and the interior casino was fun to explore a bit.

Worst: The Celsius from X-2, talk about gaudy and overdesigned.

Functionality:

Best: Invincible from FFIII, while it's slow, it also has a free inn, shops, a Fat Chocobo for item storage, and the ship actually launches fireballs at enemies during aerial battles. Not to mention it still functions as a fun mode to travel.

Worst: Ragnarok, it's honestly the slowest airship in the series. Badass design aside, the ship is actually no faster than the Gardens in travel speed. I still think it's one of the most disappointing airships in the series besides maybe the Strahl in XII.