Edea. Definitely Edea.
She's just so damn gorgeous.
How are you defining temporary, anyway? I think that, at the very least, Laguna, Kiros, and Ward are significant enough and are used long enough to be more than just temporary -- I mean, you don't control Squall and co for the whole game either
I liked using Edea, the most. Her character design is one of the more interesting, and it's always quite fun being able to have a boss character in your team. Even if she isn't quite as fun to play with as Beatrix in FFIX.
I was so disappointed that Edea wasn't a permanent party member
I liked Seifer. He had a trickier trigger window.
Laguna wins, no contest
Seifer was awesome playable, but he makes a better villain imo, and Edea was nice but a little bland imo.