There were 2 games that I have ever been addicted to, and could truly not quit playing I enjoyed them so much.
The first game was Starcraft. I could play it for hours on end, and I used to love Protoss. Zerg was probably my second favorite, but I would sit and play starcaft all day during the summer.
I used to love the custom maps, and one of the funniest custom maps I ever played was the break the condom map, which made me laugh my ass off the first time I played it
My second game that I could truly not put down was Mechassault 2. Which your probably thinking how? Well, I am not really sure why I enjoyed it so much, but I did. It was around the time of Halo 2, and I played Halo 2, but Mechassault 2 did it for me more than Halo 2.
My favorite mech as the Belial, and I would sit there with my clan (187 soldiers) all day and night playing. I haven't the slightest clue what made me enjoy the game so much, but I did.
What games could you not put down?
By the way... Mech Assult 2.... epic.
There are a bunch of games I've played huge consecutive chuncks of, but not gotten addicted to; one such would be Fable, something like 20 or so hours straight (it's possibly if you take your time and try to get everything).
Good thing Xbox 360 has made up for that with Jap and American rpgs.
World of Warcraft
World of Warcraft
World of Warcraft
.....
oh and the entire SSX series