I think my most consistently used team over the years has been Squall, Zell, and Selphie. Looking back, I'm not sure why. I guess it's because for a while they're the only team you had, and after I got the rest of the members I just kept going with it. Though I always remember thinking the "real team" should have been Squall, Zell, and Rinoa because that's who they used in the FF8 strategy guide, haha.
Whenever I take a break from the main story to do side quests or to spend time leveling up, I make sure I have the whole party on rotation.
I also really loved having a party of Squall, Zell, and Seifer for the Dollet mission. I remember making a seperate save file just for my time in Dollet so that I could have Seifer on my team and explore the town without worrying about my SeeD score. I'm also a fan of being able to use Laguna, Kiros, Ward, and Edea whenever their part of the story came up.