I'd have to say Officer Pulaski in Grand Theft Auto: San Andreas. The whole game, he's a flat-out asshole, but when I had to chase him down and kill him, his assholery goes up to 11 as he continually taunts CJ, and on top of that, he's incredibly slippery and has an absurdly high health count, making him hard to kill and therefore pissing me off even more. So when I finally damaged his car enough to force him to get out, I pulled out my assault rifle, locked on to him, and just pumped the entire clip into him. Add the fact that the very last bullet in the clip was the one to completely finish him off, and it felt AMAZING.