If you could only have ONE game in your Steam library which game would it be?

GTA IV

This

TF2

League of Legends if it were on steam.

Fallout New Vegas

Theme Hospital or GTA 1

Dota 2 ofc

Bad Rats

Plants vs Zombies.

football manager

Warhammer 40,000: Space Marine

Terraria or Don't Starve, until Starbound comes out. Then it'll probably be Starbound.

Trackmania 2™ Stadium

Counter Strike: Source most probably.

Teto

Just Cause 2

Civilization 5, definitely :p

Skyrim

Europa Universalis 3

Valve Complete Pack :3

I'm just kidding, it would have to be Half-Life 2 Deathmatch.

Actually i'm spending most of my time on non steam game. If i were to have only one game, it would be probably one i don't even get, because i know it would be a time sink for me, like football manager 2013 for example (pretty sure i could spend a lot of time playing it, since i don't have that time, i don't buy it to avoid temptation).
Finally, if i had to choose only a game i got in my library... Could have been TF2 a while ago, but now i feel the game completely broken with all the F2P stuff. So i would take NS2. Spend so much time on the first...

dota 2

Defense Grid (plus all the DLC).