I have plenty of respect for all 3 titles and more besides. I've heard great things about the physics and AI of rF2. The actual development for the software as a whole however seems to be going at a snails pace, as if by the time they're ready to leave beta we might be up to DirectX 13.
AC gets a whole lot immersive nuances across very nicely, but modders who have dug beneath the surface claim that while stock cars do indeed feel nice, things aren't really being simulated (or calculated, I suppose), such that the end goal is more important than the method of achievement. Which in many respects is true. It's hard to know where they'll be in a few years after selling to new owners and struggling to meet expectations for the console versions. Hopefully they have an amazing sequel in a couple of years.
Project CARS is an interesting one. The WMD platform both made pC the game everyone loves to hate, but it also directed a whole lot of profit directly into the development budget for a sequel, rather than mostly being diverted to publishers. So what we have now is a development studio with good history in terms of classic sims and they also have both the ambition and financial freedom to deliver something compelling. Interestingly, many oogle over GTR2 but never much is spoken of GTR1... perhaps history will repeat?
At least on PC we've never been locked to one dominant franchise, so it's a win-win-win-win-win-win-win-win-win-win-win-win-win for us :)