A party of my favourite characters would be:
Terra
Celes
Locke
Vincent

My strongest party battlewhise would be:
Cloud (because of Omnislash)
Aeris (because of Great Gospel, and I would equip either her or Cloud with the Knights of the Round materia)
Terra
Celes (I would make one of them an attacker with atma weapon, gengli glove (excuse my bad grammar but I forgot the exact name of that glove) and offering and the other a spell caster with economizer and gem box)