Civilization, Pirates, or Mount and Blade. One of those three is something I can boot up anytime and be playing until 3 a.m.

If I'm pressed for time, though, I'll go for a classic arcade game like Donkey Kong, Burgertime, or Robotron.

Oblivion/Skyrim or something casual like a time management/point-and-click game with a fantasy theme.

Might & Magic 6.

Gmod TTT or CSS

Every year I reinstall Diablo II: LOD and Jagged Alliance 2.

Quest for Glory 1 | 4
Deus Ex
Thief 1 | 3
Heroes of Might and Magic 2 | 3 | 4
Unreal Tournament 3

Dead Island.

Morrowind
Deus Ex
Grand Theft Auto Vice City
Under a Killing Moon
Anything in ScummVM

Those would be my top choices. Some modern games are becoming new "go-to" games as well, like Skyrim and Human Revolution.

Borderlands 1 and 2

HoMM III
Oregon Trail II

Minecraft or Eador: Genesis, depending on my mood. I'd say Morrowind too, but I'm trying to forget the plot of it before playing again.

Airline Tycoon, for some inexplicable reason. I usually get bored of it in a few hours, but I need those hours every couple of months or so.

New Vegas or GTA, just kill random people.

Mahjong Titans. =P

Dark Souls. Finished the game, started a NG+, then started a fresh run with another character.... more than 150 hours in and still, once a week, i feel the need to go there and do some stuff.

Also GeneRally, which i thought was abandoned years ago, and then discovered got some substantial updates recently... small addictive racing game, with tons of amazing user-made tracks