I'm sitting here at working watching the Deadly Premoniton Endurance Run - Vinny and Jeff version and God Damn is the lanky lady that crawls on the walls the fuckin WORST!!! Maybe it is how they approach it but god damn it is my least favorite part of the ER by far. Some may argue it is not a boss battle but I would disagree.
My personal worst boss battle is the last 2 bosses in Prototype. I recently beat it this past week and FUCKING A** that is the worst. I have never been more frustrated with a boss in my entire life. I was only playing on easy just to beat the game and FUCK I almost broke my control and the disc. Loved and really enjoyed the game up until the last hour or so.
By worst I don't mean the hardest even though those can qualify.