If there was ever a game I wished so much to erase from my mind so I could experience the same joy received upon first playing it then it was Deus Ex.
It is a highly atmospheric and heavily polished roleplaying shooter with similar attributes to System Shock 2. It gives players the ability to choose how they want to play with a complex skills system and many different ways to accomplish missions. You can go in the front door shooting, sneak your way past confrontations or even turn powerful robots against their would be masters.
This also gives Deus Ex a high level of replayability and every time I seem to discover something new. I tried different skills, augmentations, weapons and weapon modifications in all manner of missions and overall they all worked well and provided even satisfaction.
If the gameplay didn't provide enough entertainment the excellent conspiracy narrative and awesome cyberpunk style places you in a nefarious and intriguing world which you will explore every inch of. To complement that is an amazing series of techno music tracks which adapt to the gameplay as you enter conversations, fight enemies and explore.
Together with System Shock 2, Deus Ex shares the spot as my favorite two games ever. I've played to an utterly insane degree because I keep coming up with new ways to beat missions. And when I jump off a ten story building and slice an MJ12 trooper with a gigantic nanotech blade, I can't help but smile.