Mario Kart, because driving sims bore me to tears.

Project Gotham Racing.

I still have to get around to buying All-Stars, evertime I intend to another game pops up.

The Real Driving Simulator Gran Turismo.

I'd say...
MK. Never been into GT.
The only racer I truly played for hours was MK: Double Dash.

GT for it's variety

Gran Turismo, there's nothing that gives the same satisfaction when you do well. Then there's the variety, from the mix of fantasy track and real world. Rally, nascar and even carting and F1. Time of day changes to racing in a down poor, nothing matches it. The sim wheel selection for the game is amazing as well, from your budget wheels to top of the line.

Mario kart is fun. but if i had to choose. I'd go with GT.

I know what you mean, so many games, so little time.

I have had Crysis 3 & Bioshock Infinite sitting here for over 2 months both gathering dust, still to get fully in to the likes of Arcana Heart 3, Deadpool, Mark of The Ninja, Nights & Anarchy Reigns. (and thats just the 360 stuff)

Had a taster of the following ps3 games, but still need to sit down and put the hours in properly.... UC 2&3 HR, MGS4, KZ 2&3, Heavenly Sword. (+ many more)

Completed UC1 however, completed GOW Chains & Ghost, and im a good 60% of the way through GOW3.

Mario Kart.