The list as I recall is:
Seifer
Laguna
Kiros
Ward
Edea
At some point during one of my playthroughs I made a separate save file for the Dollet mission so that I could go back and play as Seifer whenever I wanted.
I always enjoyed the Laguna flashbacks and was very intrigued to see Edea use her limit break on someone that wasn't me!
Who did you enjoy using. Did you want them to join the party permanently? Who would you have liked to play as?
I would have liked to see Fujin and Raijin even for one battle. I imagine a scene where they turn to Seifer and say "enough is enough" followed by a boss fight where they join Squall.