My vote went for Penelo. She's not actually as useless as the rest of these guys and surprisingly makes a really great black mage, but in terms of the story she has no purpose whatsoever.
Yeah, Cait Sith is pretty well tacked-on and has a flimsy pretext for joining the party, but he at least has a pretext being that it's actually Reeve controlling a robot, and he actually does something useful in retrieving the black materia.
Yeah, Quina is completely tacked-on and has no motivation that I can discern that's relevant to the story, but at least he/she is amusing and serves as a colorful and unique addition to the cast.
Yeah, Rikku has horrible dialogue, but she at least has a personality (if an obnoxious and overly cheery one), she at least has unique and useful skills, and she probably has the strongest role in the story of any of the characters listed here as she is Yuna's cousin.
Penelo? She, like Vaan, just sort of got roped into the whole debacle of FF12 by circumstance and honestly has no business being involved with the heroes after a certain point; she could easily leave the group and nobody would notice. Visually she strikes me as a hollow imitation of Rikku, and personality-wise she just... umm... ... She... .... what was she like again? Meanwhile the skills I can attribute to her are only such because I made her a black mage through the crappy license board system, which I could've done with just about anybody. She just... isn't interesting, even when you compare her to Quina, and that's a tough sell.