Ok, after messing with FF anytime, I've come to the following conclusions:
1) Obviously, not all moves should be FF'd
2) However, there are a few that could work as character buffs. I'll only list certain characters (ones that could probably stand a buff).
Link: FF upB is actually pretty cool. He can grab the ledge on the way down, unlike Sonic's FF upB (that made me sad). He can use it from the ledge to clear out the opponent with upB and regrab with the fastfall.
Ike: FF sideB is also cool. You can full hop, then FF into sideB, which will come out just as you land!
Bowser: FF upB would help him get down when he has to recover high. FF after the bowser slam can help him tech chase. FF downB actually slows you down, since the regular downB is faster. Mindgames?
Samus: FF missiles are godly, nuff said. (BTW Sam gets a lot more out of the FF ziar than Link does). FF downB is actually a cool way to have a bomb fall right in front of you as you land and do any attack behind it! Just full hop and FF right after the downB. Her FF upB works like Links, and could probably be described as broken. Samus is the undisputed FF special master.
Ivy: FF razor leaf is pretty coo, but I don't think Ivy needs much more buffing. If he/she does, this would be a good one.
Chari: FF Rock is awesome, as you would expect.
Jiggz: FF pound is pretty cool. Not amazing, but makes it better.
Yoshi: Doesn't get much, but FF egg can get you out of the air. His downB acts just like bowsers, but is easier to notice since he is smaller.