The Seraph - Master Lamington, in Disgaea: Hour of Darkness was a good challenge for me. Fighting just him isn't all too hard, but the six other angels you have to take down make it all the more difficult to deal with Lamington. Depending on how well-prepared you are, by the time you're ready to deal with him, you've already lost a few of your characters. Felt good when I finally beat him though; I had to do a good deal of levelling up to win, but even then it was still pretty difficult.
Heck, alot of the in-story bosses in Nippon Ichi's games make you think on your toes. I remember almost all of them having a good level of difficulty to them. Still, barring the side-quest bosees and the like, Lamington takes the cake. Tough one, that guy is.
Hardest boss ever? Maybe, maybe not. Toughest one that's coming to mind, though.
