Call of duty: Modern warfare 2 I played for about 14 hours when it first came out. I spent a little time on one player but after I beat it I went straight to the online play.
It doesn't really specify, but overall the game I've played the longest would be Grand Theft Auto: San Andreas. When I use to have this game and my PS2, I had over 20 pages or so of cheat codes and it never got boring.
That game was too long for me. I never beat it. I got to like the 3rd map. I couldn't get any farther then that. My cousin and older brother beat it though. The most time I put into that game though was like 2 weeks. I played it every night for a couple hours then just gave up when I couldn't beat it.
I prefer to play GTA4 since it was shorter and easier to beat. I liked the graphics and features better on it too.