As a kid I used to play even bad games for ages and played the same game over and over again untill I finally got a new game. Now that I have more money and a lot of unplayed games lying around, I have hard time even finishing/getting into great games.
Online games would be
Counter Strike Source
Day of Defeat Source
Half Life 2 death match
Played all these 3 since 2005-2006 irregularly. I might play nonstop for 2 months and then not play for over a year.
Warcraft 3
the same thing
And I played World of Warcraft for about 350-400 days of /played time, whats that, 9600 hours of real life time? Although I had a lot of breaks as I got tired to the same old thing several times and I havent played for a long time now. But I do miss the game, like I always did after having a break longer than 6 weeks.
I wonder how much people who have played from day one till now hardcore (8-18 hours a day 365 days a year) have got /played time

I also kind of want to go back to some of the other MMORPGs I have played but some of them are dead with no servers left.
Its harder to list offline games since there are so many great games that I like to replay every now and then. But since I have so many single player games waiting to be played lying around, its hard to say that I never get tired to some titles since there is so much time between my playthroughs that it never gets tiring.
But if I have to name a few, I would say that I never get tired of playing through the story of Half Life 2 again and again. Tony Hawks Pro Skater game series, when counted together, is the console game that I have spent most time playing probably. I have completed 1-4 and Underground 1&2 all several times 100% and played countless hours of freeskate.