LFS, without a doubt.
Although GPL was teh bestest when it came out, it's physics are still among the best ones around, but it is dated. Low Speed grip is way below par, and the sliding that can be done with it is unrealistic. Proof of that is in the difference in fastest laps between mere mortals and "teh aliens".
IMO, in a sim that has ultrarealistic physics without anything that makes it any different from reality, the 'aliens' would not exist. There'd be slow and fast drivers, but they'd all be going for the same limit, the limit of the car. As it is now, limits in the physics engine get exploited by the aliens. LFS doesn't have real 'aliens', just guys that are faster than others. If you compared the average driver's times to the Pre-April-patch physics to the post-april-patch physics, you'll notice that the slower ones are actually closer to world record times, because the physics behave more like real world.
Also, physics is the 'only' thing I want in a sim.