I'm sure that, among current games, World of Warcraft has the undivided attention of many of the world's gamers. However, I haven't even tried the trial version .... and won't! I'm just not that interested in MMO games, FWIW.
But, for me, I'd have to say I've spent the most time on three different games over the past few years:
1.
Oblivion and all its 'sequels' e.g. Knights of the Nine, Shivering Isles, Lost Spires, etc. I know for certain that I have close to 700 hours devoted to this game.
2.
Morrowind and all its 'sequels' e.g. Tribunal and Bloodmoon. This, too, would be somewhere in the hundreds of hours.
3.
Star Trek: Deep Space Nine -- The Fallen. (I even worked with one of the developers making a free add-on a couple of years ago called, Convergence. The name was even my creation.

)
So, which game (or games) has taken up the most of your gaming time?