GTA =)

Too many to choose from.

Half Life Series, Mafia:The city Of Lost Heaven, Hidden and Dangerous 2, Call of Duty 1 + United Offensive. Call of Duty 2, GTA Series ( I love Vice City), Company of Heroes, Rise of Nations, Oblivion, Skyrim, NHL, UFC Undisputed 3, Fight Night Champion, Brothers in Arms, Silent Hunter III, Fire Department and many more ... But Half Life series and Mafia 1 is just my top ... Hidden and Dangerous 2 also ... I grew up on those games 

Well if I have to choose only one game: Star Wars Knights of the Old Republic. 

If this is going by gametime, Battlefield 3 would be my favourite game with over 3000 hours. 

In general though, I don't really have a favourite. I play War Thunder a lot, but I usually end up raging. 

You want me to whittle it down to one?! Don't be insane...

If I was being really picky though:

1. The Elder Scrolls: Skyrim - played it since day 1 and most of my off days are spent playing it
2. Star Wars: Battlefront II
3. GTA: San Andreas

 

I play War Thunder a lot, but I usually end up raging. 

And taking it out on me...

*hands up don't block*

Right now the game that has me hooked is ARK: Survival Evolved.

The Valve games, TF2, Portal 2, and most recently, CS:GO all have such fascinating communities and gameplay options. 

There are far too many to list

 

Burnout

Burnout 2

Burnout 3

Colin McRae Rally 3

Colin McRae's DiRT

Colin MCRae's DiRT 2

Corvette

DiRT 3

Driveclub

Driver

Driver Parallel Lines

Driver San Francisco

Forza Motorsport

Forza Motorsport 2

Forza 3

Forza 4

Forza Horizon

GTR2

Gran Turismo

Gran Turismo 3

Gran Turismo 4

Gran Turismo 5

Gran Turismo 6

Grand Theft Auto III

Grand Theft Auto Vice City

Grand Theft Auto San Andreas

Grand Theft Auto IV

Grand Theft Auto V

Hitman 2: Silent Assassin

Hitman: Contracts

Hitman: Blood Money

Juiced

Need For Speed Hot Pursuit 2

Need For Speed Underground

Need For Speed Underground 2

Need For Speed Most Wanted

Need For Speed Carbon

rFactor

Race 07/GTR Evolution

Saint's Row 2

Scarface: The World Is Yours
Race: Driver Grid

ToCa Race Driver 3

Tom Clancy's HAWX 2

Tom Clancy's Rainbow Six Vegas 2

Tom Clancy's Splinter Cell Conviction

 

This is the games I know off the top of my head. May update this list.