In order of preference ...
1. Cecil ... while I admit He's not the best choice for a rep, he's my favorite choice. I have very fond memroies of Final Fantasy IV back when it was still Final Fantasy 2.
2. Terra ... much like Cecil, she represents the classic era of Final Fantasy and is honestly a better design with more move potential... I'm just not as attached to the character.
3. Black Mage ... it's harder to get more Final Fantasy than a Black Mage. He gets knocked down a few points for not being a actual character but a class, still he's more iconic than pretty much any of the people on this list and would fit well into Smash's lineup.
4. Yuna. Not exactly a favorite character of mine, but plenty of move potential and an interesting look. Ideally she'd draw on moves from both X and X-2.
5. Squall. While I hated a LOT about FF8, there was still something likeable about it's cast and Squall has a nice sturdy design to him and an interesting weapon.
6. Zidane is fun. Really fun and possibly one of the best main heroes Final Fantasy ever did. He's just lacking a bit in the iconic status.
7. Warrior of Light ... if you mean the big point helmet redesign Final Fantasy hero, then No. He doesn't belong on the list at all. If you mean the red Fighter character, he could be decent, even though he's rather generic.
8. Sephiroth just really doesn't seem like he'd fit into the series at all. He almost feels like the exact opposite of what a Nintendo character would be. He ranks eighth place because everybody lower on the list I either dislike or just don't care about.
9. Onion Knight. Cute but bland.
10. Bartz. Bland without the cute.
11. Vaan .... anime Aladdin here didn't even belong in his own game. He really doesn't belong in a collection of all stars.
12. Noctis. I have no opinons whatsoever about Noctis.
13. Lightning. She always struck me as Square trying a bit too hard and not coming up with anything interesting.
14. Tidus. Winner of the MOST Annoying RPG hero ever award and still the character I use as an example when i talk about poorly designed JRPG characters. There's not even names for the stuff he's wearing. Seriously. Try describing his outfit. A quarter of a hoodie with rubber overallcorset and detachbale shorts leg lenghtiner?
15. Firion exists.... and that about all you can say about the guy.