I'd vote for Lulu because she's my favourite character in FFX/X-2, but she's (SPOILER)a full-time mother now, so gallavanting and adventuring wouldn't be the most responsible thing for her to do.

I'd have to nominate Brother. After all, the guy's a part of the Gullwings already; he's related to both Rikku and Yuna, so it'd add to the "family feeling" that they've already got.

Also, by working closely with Yuna, it might help him to overcome his feelings for her. "Too much of a good thing", you might say. I think he'd get over her more quickly if she was no longer so out-of-reach, away on journeys while he's stuck on the airship.

Also, Brother would probably be a pretty handy team-member. Sure, he seems like a bit of a goof, but he's fit and strong, and pretty resourceful too. You can see this from his prowess in Blitzball, and his experience as an Al Bhed excavationist. He's probably got as much adventuring experience as any of the others.

Edit: I wouldn't want to see him using the Lady Luck dressphere, though :S