Out of the few I've played so far, IV would come in close. I've had it for almost 3 months and haven't touched it since a week after I'd gotten it because the ending (and nearly all of those stupid bosses) was too difficult for me, and some days I just wanted to throw it at my wall. :|
But, I'm willing to forgive...so II, I guess, would be my least favorite. The "leveling" system is just way too screwy.
