D2 - Final Boss. Not hard but the concept was piss-poor and the execution was even worse. It was annoying , very annoying.
Ninja Gaiden - Alma. Not sure why, but the first form of this boss caused me many problems.
Resident Evil (PSX) - Tyrant
It\'s odd, I have played so many games and no bosses really stand out as being "hard". Yet, I know for a fact that so many have caused me many headaches.