Diablo was the first ever game I played, so it has a kind of sentimental value that makes it enjoyable to play.
Mount & Blade was the first game I bought with my own money, and the modablity of it is incredible, so it is a good game.
The Elder Scrolls IV: Oblivion is THE MOST EPIC GAME ON THE PLANET!!!
The Sims 3 is a very new and astounding game (Yes I have it. Bask in my glory!) with SO many new features and improvements from The Sims 2.
Faces of War is a very good game, and surprisingly addictive. It's also fun playing the game using the GEM Editor for obvious reasons.
Call of Duty 1, 2, 4 and 5 are probably the games that have had the most playing time. I don't know why but the replayability factor of any CoD games is just spectactular, and the multiplayer is so fun, and there are still so many players in the earlier games.