Manhunt looked pretty violent from the screenshots I saw. I never got around to playing it myself though.
I don't really play "violent" games much because they are usually third person shooters/slashers. I generally only play games like Half-Life, Counterstrike, and Battlefield, all of which are somewhat violent but nothing to rank as "most violent." Half-Life and Half-Life 2 get pretty violent in some spots but I'm sure not anywhere near as much as some other games..
For me the most violent game I've ever played was Soldier of Fortune 2.
Even though I don't really like shooting kind of game but for Soldier of Fortune 2 was an exception. As the matter of fact the reason I play the game because of the detail how much the enemy will bleed from my shoot.