Simple question: What is the MOST DIFFICULT Boss that you have ever fought in a game?
No matter a fighting game, MMOG, action-adventure, FPS, anything.

The term 'HARDEST' does not necessarily mean that you had never won against it.
Of course it can, this thread will be going that way too anyway
Sometimes, gamers intentionally tune up the level of difficulty to the max just for the thrill or it

'HARDEST' in this term, is a Boss that seriously drive you nuts to beat it (whether you did in the end or not), up the point of:
- searching for a cheat code
- ruining your mood
- breaking your controller
- ranting in a forum of how much you hate that Boss, and the game company that make that game