VI - Terra, Setzer, Strago, Gogo
VII - Vincent, Yuffie, Cait Sith and, to a lesser extent, Cid and Aeris.
VIII - Irvine, and to a lesser extent, Selphie & Quistis
IX - Amarant and Freya

I find X, XII and XIII really benefit from levelling everyone up so I tended to rotate a lot more often. I probably used Yuna/Penelo/Hope least in those three games just because I wasn't a huge fan of them.