Quina. Quina can kill almost everything in the game with one hit until about the end of disc 2. The all s/he needs are two hits and not to be attacked inbetween. You don't get much better than that.

However, that's only if you're playing a speed or lv. 1 game. Overall, I find Vivi most useful. He can, from the very beginning of the game, attack every enemy at once, and he's pretty consistently powerful. He makes Steiner become more powerful, and when you couple him early on with Garnet and she casts reflect on him, he's even more powerful. And he's so cute