5. Hell - Doom 3
This game is one that will stay with me forever, due to scaring the hell out of me when I first played it back when I was about 12, in the dark, with headphones in. The whole game was creepy, due to the flickering lights and surprise attacks by the minions of Hell. The actual base itself was huge and was littered with destruction, from faulty doors to eviscerated bodies lining the hallways. We all knew Doomguy was going to have to travel to Hell at some point too- in keeping with the original Doom. What followed though was one hell of a scary ride (pun intended.) The Satanic imagery was more pronounced and in your face and your enemies took on a whole new grisly form, since you are fighting them on their turf now. Let's not forget too the fact you take on the brand spanking new Cyberdemon in the bowels of Hell too- just to make it that tiny bit more scary.
4. GLaDOS Boss Fight - Portal
After being pitted against a series of tests in the Apeture Science Enrichment Center by everyone's favorite murdering supercomputer- Chell, our protagonist, finally confronts GLaDOS face to monitor. GLaDOS has personality though which made her one of the greatest end of game bosses ever. Chell is tasked with removing and incinerating all of GLaDOS' personality cores. During the battle though we learn that GLaDOS was responsible for killing everyone in the Enrichment Center after releasing a deadly neurotoxin. By no means was it the hardest level ever- but it was the continuous commentary from GLaDOS that really made the level stand out from the constant puzzling. The one let down of this level however was that you never did get your cake she promised you. It's not a lie though, promise!