Probably Erza because Jellal wouldn't hurt her because he loves her❤❤❤. She has that lightning empress armor, so Laxus wouldn't be too much of a theart. Almost everyone else is scared to death of her, but Mira might be somewhat of a threat with her take over power. But I think Erza could take her. The only person I'd be worried about is Zeref because he can kill anyone with that curse of his by accident. That's one of the reasons I wouldn't choose him as my partner because he might accidentally kill me, and Natsu would probably take down Zeref with his abillities. Clearly, I have thought about this too much, but Erza is my choice.