~ Quina's a pain in the ass, though useful when it has all its skills. (Having to weaken/eat enemies is a major drag, though, FF7's enemy skill system was better.)
~ Garnet is unreliable for reasons already mentioned in this thread, though she does come in handy, especially early on. (Later, Amarant's Chakra, Freija's Reis' Wind, and people using Potions usually gets the job done.)
~ Vivi is probably the best; though he's weak physically he has some strong offensive magic and will probably be the brunt of your offense most of the time in the final area (that tree place). Auto-Reflect + (magic)2 on all allies = 9999 for enemy most of the time.
~ Eiko's a weak knockoff who is mostly there for storyline purposes and should've been killed off, IMHO.