Who is a Boss that gave you a really hard time? (Any game)

Artorias the abysswalker from Dark souls... I managed to kill every boss in that game with a max of two or three trys (ok ornstein and smiugh maybe took five...) but Artorias... I had to change my playstyle completely to beat him...

Dead or Alive 4, the frigging last boss.. The living room has a hole in wall it is a reminder of how annoying the boss was ;D I pretty much saw red with that boss, died like 200 times to it :D