Meiguoren

The hardest bosses I have ever come across have been Chaos from the first Final Fantasy (2000 hit points and liberal user of magic that FULLY restores him) and Zoda from both StarTropics games. While he was rough in the first one, he's even harder in the second. I haven't been able to beat the second one yet. And then there's the old standby, almost any form of Dracula from Castlevania (the exceptions being Castlevania II and Symphony of the Night).