Forza 4. I am playing this since the day of its release in 2011, and there are still championships to beat and cars to be purchased through prize money. I think this game won't only fulfill another summer worth of playing, but rather some years.
I still need to complete that 100% but despite being very solid technically it's the lack of track variety that brings it down.
Agreed, some more fictious tracks apart from all the real world race tracks and their numerous variants would have been nice. As well as rain, snow and night races.
But for what the game does, it does it on an excellent high level. My brother is a car nut that played both the Grand Turismo and Forza series, as well as some other racing sims on PC, and he says he would still prefer Forza 4 amongst all of them.
For myself, well, it is the only racing sim I could ever get used to and even drive without any assists (apart from the rewind feature) now to get maximum credits for won races, since some legendary and high end vehicles are outrageously expensive. Good thing the Xbox 360 controller seems to be well-designed for racing thanks to the left stick's position and the analog trigger's shapes (things I really miss when holding a PS3 controller in my hands). At the moment, I try digging into suspension and tire tuning (apart from just attaching hop-up option parts), as well as doing my own paintjobs for some cars. The tuning opens up a way to get even bullheaded or twitchy cars much more well-behaving.
When playing, I mostly stream music from my laptop's library to the Xbox 360, e.g. listening to Outrun's Splash Wave while driving a Testarossa through the Bernese Alps.
By the way, what's your Xbox Gamertag, so I can add you to my friend's list and compare track times? EDIT: Nevermind, just found you in the gamertag thread.