it depends really. i gave ffviii my vote because it was quite hard when i first played, i didnt unlock limit breaks (the dungeon was quite perplexing) and didnt have anyones final weapon. i got by by using triple and casting tornado alot.

ffv is good if you start at the beginning of the cleft of dimension. Its boss after boss including the two optional bosses right up to X-death and Neo X-death. Thats very enjoyable to blitz them all one after the other