As brief a list as my rant-loving brain can muster:
-All the bosses in Dark Cloud have ungodly long health bars. Once you get the pattern for the fight, they'll never hit you and it becomes a waiting game. The only exception is the optional boss at the end of the optional dungeon.
-Night-Gazer Khighorahk from RuneScape's Dungeoneering skill. His health values are just retarded and he's the easiest boss I've fought so far in the skill. This is made worse by the fact that the boss at the end of each floor is pulled from a random table and you will have to fight him many, many times.
-The final boss from the original Gears of War. My first playthrough was on FUCKING CASUAL(!) and he brutalized me in ways that I don't want to talk about and should probably tell to a therapist. Although that may have been due to playing alone in a game that is just plain made for co-op. Dom's AI is too dumb to live in solo play.
-The final boss in Prince of Persia: Sands of Time and the first two in Two Thrones.
-The optional bosses in Kingdom Hearts. They're all just brutally huge health bars.
-Also I feel like the colossal pieces in HeroClix deserve an honorable mention. Fing Fang Foom, Dr. Manhattan, Galactus, The Specter, Sinestro, Starro, The Sentinel and Monitor/Anti-Monitor rank among their numbers. And they're just brutal. Except maybe Sinestro. He's not too impressive to be honest.