Since you can't de-select Squall outside of a few key moments of the game (lunar when Zell leads the party, Selphie's missile base team etc) I think he's kind of a given. I like to team him up with Irvine (limit break, give him an Exeter and let loose with pulse ammo in boss battles) the 3rd slot in the party is quite interchangeable as many have said though Selphie is a common choice because of her limit-break so long as you have ATB set to wait you can pretty much scroll through until something awesome (The End for example) comes up.

Personally my favourite party though is; Laguna, Kiros and Ward! God damn they kick ass.