The Elder Scrolls 5 Skyrim
The Witcher
Lineage 2
World of WarCraft
Gothic 3
Fallout 3
whatever, but for me was probably the Witcher.
Civilization 5, Quake 3, Heroes of Might and Magic III, FF Tactics, S.T.A.L.K.E.R.H., and a bunch of old PC only RPGs.
Heroes of Sword & Magic
Disciples
Unreal Tournament
Mount & Blade
NFS Underground 2
Tekken 6 (PSP)
If we don't consider multiplayer, then The Elder Scrolls 4: Oblivion. Those were the days... Explored the whole world.
The games of the '90s to 2010. Those were really interesting. Now only a couple of normal games are released every couple of years. Now I hope for a new Mass Effect. I hope it will be something in between 1 and 2. Another hope for a couple of games, but they haven't been announced yet.
There's a difference between playing something all the time, like DOTA, CS, and between running a game and lagging 20 hours a day, games like Civilization 5, Jagged Alliance 2, Euro Truck Simulator 2, Stalker, The Binding of Isaac