Very rarely do I find that they come out with a definitive version that I like best, but I'll try my best for brevity,

FF1 - FF:Origins. Keeps the original difficulty but improves play-ability.
FF3 - DS
FF4 - DS increased difficulty and my inclination towards 3D blocky chibi sprites makes this a win.
FF5 - Only have experience with SNES
FF6 - Only have experience with SNES
FF7 - Tough call. PC is an all round better experience, minus the accessibility of the PS3/PSP combo.
FF9 - PS3/PSP.