Knights of the Round (ffvii), I freaking love that one, they just keep coming, honourable mention to Hades as I always forget about it and enjoy it when i get it.
Siren (FFVIII) was pretty
Alexandra (FFIX)
Anima or Shiva (FFX) the summons were great in this game
Ultima (FFXII) pretty skirt, but i love all the espers
not sure on FFXIII yet as i only have three so far