The Half-Life series. All of them. I love them and can't wait to see what comes next.
Elder Scrolls Oblivion is an awesome game as well. working on playing it through a second time with a different character.
Titan Quest and Titan Quest-Immortal throne Excellent game. Kept me occupied playing all sorts of different characters for many evenings
All those early FPS games like Doom and Wolfenstein 3d. I played the hell out of them and have been addicted to that type of game ever since.
Mario Brothers, The original. I still bust it out and play sometimes.
We used to stay up all night and play the mega-man games as well. We would rent them and then play all night to beat them.
Anyone remember RadRacer? man I loved that game!! I got some serious blisters on my fingers playing that!