Postal 2.
No other game lets you tazer a cop to the ground, pour gasoline on him, throw on a match, watch as the cop turns crispy and before he dies piss on him to put the flames out. Then cut his head off with a spade and kick it out of the window and into the crowded street below, all of whom panic and call the national guard, but before they can machine gun you down (and rightly so) you smoke a bowl of catnip, yell "Yeah baby, I am the lizard king" and start booting (duke style) your way through the crowd. Next you find a hiding spot in a convenience store. A sign tells you not to go through the back, but you do so anyway, fleeing the cops. Out back you are rushed by the store owners legion of wives (screaming 'Allallalala')...need I go on?
The game is only as violent as you make it, but there is next to no limit (apart from your own soul/sanity) as to the depravity and violence once you start to put a little thought into what you can do.
Heres waiting on Postal 3 and the Postal movie. Neither will live up to my expectations...but if they do i'll be a happy chappy indeed.