The most powerfull limit break is Zell Duel atack (Armaggedon Fist)

I have:

Zell= because he posseses the most powerfull limit break and his my favourite character

Squall= because i´m forced to use him

Selphie= i use her because of Full Cure and mainly for support character plus she is my second favourite character

If i could i would change Squall for Irvine since his limit break his more reliable.