Terra, cause she fried fifty magitek soldiers in a few minutes, and mechanically, she has the most build potential out of the group whereas everyone else is kind of limited.

She's also referred to as one of the strongest beings in the game, I mean that has to account for something. She can just fly to the top of the cage and rain down magic death on all of them, not to mention her huge power boosts in Esper form, even just using in game mechanics gives her an advantage unless everyone teamed up to fight her first.

Personality wise, Cyan is too honorable to hit a girl, he sure as hell wouldn't let Sabin hit her and I doubt he would want to since he's too nice. Shadow might actually try something but technically he's outmatched since Terra isn't exactly a push over fighter and all four of them will be too busy taking down Umaro anyway since he is only loyal to Mog. after Umaro is gone, I'm pretty sure the three boys will gladly take a dive for Terra since she's pretty sweet anyway. Terra probably wouldn't even fight unless she goes into a rage when she feels they are picking on Umaro too much. I don't think any of them would stand a chance if she was angry and actually trying.