The hardest boss you've have ever fought?

[Metal Man]

Slarvax said:
Dracula from Castlevania. Took me way too many times. Cant think of any other.

 Same here.  I've only beaten the original Castlevania once, and I died 152 times against Dracula.  It took me 189 lives to beat the game on that playthough, so I spent the vast majority of my time fighting Dracula.