too many to choose from, maybe Fallout (yes I'm talking Fallout 1) or PST. If you want real old school then its The Lords of Midnight (zx spectrum) one of the first games I ever played. more modern games Saints Row III and FTL are current favourites.

Pepsi Man ! jk

Far Cry 3 or Assassins Creed III

I'd say Silent Hill 2

Deus Ex.

Fallout 3 in close second.

Gumboy tournament... best game ever!
Just joking... killing floor.

Does soggy biscuit count?

DOTA 2 and Chivalry: Medieval Warfare

espectate on idle servers

counter strike series

Super Mario World, i actually just played it again.

Edit: "played it again" might be an understatement. I completed it in a day, all 96 exits, and then i went and started a new save and played till i got around 50 exits and started getting "bored" enough to force myself to stop.. :D

killer7

atlantica online! :P

Depends what genre. Strategy - Age of Empires for the nostalgia, Civ V otherwise.
RPG - probably Oblivion; first truly open world rpg type game I played (and I'm not counting Fable TLC as open world but had that before Oblivion)
FPS - Borderlands
Indie - toss up between Limbo / Binding of Isaac / Super Meat Boy and Realm of the Mad God.
Driving / racing - Driver San Fran and DiRT 3 for racing (only other decent racer I have is Shift 2 and I much prefer DiRT 3)

Chivarly for sure !

All time,probably Gothic 2.

Currently I'd have to go with Awesomenauts.

Warcraft 3 and the expansion!

The Mount & Blade series! <3

world of goo.

At the moment it's Max Payne 3.

Wizardry 8, System Shock 2, Little Big Adventure 2. Sorry for three games, but they're equally perfect

Ping-Pong

counter strike 1.6 , counter strike : source

Assassin's Creed Brotherhood