My favorite for each member is...

Cloud - Omnislash (duh)
Barret - Catastrophe
Tifa - Final heaven
Aeris - Great Gospel (or Fury Brand)
Red XIII - Star dust (or Earth Wave)
Cait Sith - Dice (Slots can kill you)
Cid - Highwind (both his level 3's aren't bad)
Yuffie - Doom of the Living (or All creation)
Vincent - Chaos