Fallout 3 and Fallout New Vegas: These two games have so much to do, I haven't even finished my Good karma run through of Fallout 3, I've spent soooo much time into it
Mass Effect Series - Ending and misgivings of ME3 aside, this series is arguable one of the best in game story-telling ever, alongside with RPG value.
Kotor 1 and 2 - Mostly 2 due to various playthroughs of the new versions of TSLRCM
The Sims 2, 3 - Come on, simulated life man, friggin awesome
Jedi Academy - This is mostly my sandbox game where I spawn 10000 desanns against my personal God character that I created. Never gets old
Gmod - Virtual reality here I come!!!!
Half Life 1, 2, Black Mesa, and episodes - IMO best gaming franchise ever, hands down. Gameplay is awesome and storytelling equally great. Though Gabe Newell needs to learn how to Count to 3