Ruler of the Seventh Empire
RAAM from Gears of War is the hardest boss I've gona against recently. I heard it's impossible on the highest difficulty to beat him by yourself.
And even on the lowest difficulty he's turbo hard to beat.
Ehh RAAM's alright in my book. So long as you stay in cover and don't miss his face with those precious sniper bullets, you're fine so long as you keep some distance between you and him. He's very close to being impossible to solo on insane mode, though.
Boss I've had most trouble with is the Wizard of Yendor from Nethack. Rodney's not hard to kill, it's just that he'll keep respawning forever once you've encountered him once. You've got to deal with him chasing you everywhere, using touch of death spells and summoning nasty monsters all over the place (possibly demon princes even, including Demogorgon, the hardest monster in the game to beat in a fight), stealing the amulet you need to complete the game and then teleporting off somewhere with it, randomly cursing items in your inventory. He will continue to do this for the rest of the game. He's a real bitch.