While we certainly have a number of capable team members, I've noticed that no one is really well rounded enough to lead every aspect of the team. We all have our pros and cons, so I think Wraith's suggestion of multiple leaders is the most logical way to go with this.

Rhea/Cell: She's certainly the most caring of the titans. Her heart of gold and tendency to give braveheart speeches makes her a good candidate for leadership. Yet, she lacks any tactical ability and has a phobia that would hinder her greatly during combat. I'd elect her for a more political leadership role in the team, as their public face and moral center.

Simone/Pistolera: Simone's the closest of us to Rose. She obviously has some sort of military background, and has shown to ability to develop on the fly tactics under the stress of combat. But she's antisocial and keeps secrets, as well as lacking any real deep connections with her teammates. Pistolera would make a decent tactical leader. She'd be our Batman, essentially.

Accelerate/Amazon: Both warriors in their own right. They have the bravado and skills to lead their teammates during the rigors of combat. Accelerate's serious demeanor leans itself well to making cool headed choices. Amazon was raised on an island of soldiers, and shows that he has no fear to disable him during battle. They'd make good field leaders for Alpha and Beta teams. However, they both have major issues they need to work out before I'd say they were ready to lead the team in its entirety.