Well, he's not really hard at all if you're high enough level, but the first time I fought Sephiroth in FF7 i was desperately low in level. The whole battle took literally one and a half hours (probably would have been 20 minutes if it wasn't for him casting supernova every other turn) I finally got him with two guys dead and Cloud with only a few hundred hp left.
Other than that, no bosses in newer games can compare to the controller breaking difficulty of some 8 bit game bosses. Castlevania and Mega Man were especially hard for me. What about that part in Meda Man where you had to destroy those cannon things until you ran out of ammo, then die, and destroy the rest next time? Thats just not right.
