FFVI rules. Albeit 2D graphics you must consider it when the era of SNES it was released it was the best graphic, music, & features includes the first perspective world map in SNES RPG. It has the best storyline & sidequests and becomes the pioneer of later FF story. So good even FFVI can be considered equal with their later 3D FF series in PSX.
My favourite is FFX... it has a nice storyline, great characters and beautiful graphics... It's closely followed by FFVIII... I haven't played VII yet... so let's wait
FFIX is definently my favourite, first one i played and ive played all of them now (except XI) and it still seems to be my favourite, with VII comin in close
For me FFVI would be the best, VII and V coming close behind, those three are definitely amazing games, young gamers could have to get used to old style graphics tho, but well, just go and play those !