Best is a tough call between FF8's Ragnarok (because it's sci-fi as hell and i love sci-fi, also it looks cool) and FF9's Invincible (because it's pretty and oh yeah it holds souls and enslaves eidolons).

I'll give the edge to Invincible for now, I guess.

Worst is also a tough call because 1, 2 and 3's airships (the Cid ones, that is) are equally bland. I'll give it to 3 though, because 1 has the excuse of being the first and 2 has the dreadnought