Well, I'm sure someone'll disagree, but I'd have to go with the final boss of Terranigma. The advent of the 'save state' feature mad him (somewhat) easier, but before then, he's nearly impossible. Get this, he launches these myriad attacks at you, each doing obscene amounts of damage. Then he launches an attack that is impossible to dodge and drains away half of you hitpoints for a long time, eventually bringing you down to one hit point, regardless of how many hit points you've got, it is also one of the few times that you can hit him.. To top it off, his defense is so high that you can only do 1 or 2 points of damage to him on a hit, so you have to sit their pounding away for a good hour before he finally dies. Oh well, I guess if it were any less difficult I'd be disappointed.