My main three characters are Cloud, Vincent and Aerith/Tifa.
I think Cloud is the strongest character in the game. He has powerful weapons and limit breaks.
I also like to use Vincent. His Limit Breaks are useful in the first 2 discs. However when you reach higher levels, they are outmatched by the Limit Breaks of other characters.
My 3rd party member is either Aerith or Tifa. I use Aerith when available because she is a good healer. However, in times when she is not available I use Tifa instead.