I vote Lulu. She's calm, collected, and the only member who's half-way decent at magic. You're in a pickle if you find yourself against an elemental without her. ^_^
Besides that, Yuna's too stuborn, Rikku doesn't do much for the team (I'm pending for a name change as we speak :P ) and Wakka drives me nuts. I hate 'im, ya? ;)