Garnet is only really useful when she starts getting her eidolons. Eiko at least has a powerful offensive spell to make up for the sort of lack of offensive eidolons. Vivi is rather useful, as he is the brunt of your offense for the first half-ish of the game. His attack sucks and I find he uses MP way too fast. So the prize goes to Quina. Quina's blue magic is awesome, and I'm doing a playthrough of this game right now and have realized that quite a few enemies succumb to the Frost and Mustard Bomb spells. So Quina can cast Frost on, say, a Red Dragon in the Mount Gulug area. It will freeze, then you can have, say, Vivi attack it and it will die with a single weak attack. Oh it's wonderful. You can get a quite a few good Blue Magic spells early on. A really easy way to defeat some enemies is to kill Quina, and use a Pheonix Down on him/her. If it only gave him/her 1 HP (which has a good chance of happening), then s/he can use Limit Glove and you've won. Quina is cool, not to mention s/he can have a high attack power.