How about all 3? All 3 have here and there great physics.
iRacing is lightyears ahead in terms of rigid body dynamics and suspension physics, not to mention the kinematic effects of collisions are the best I've ever seen in a game. All of these things are Papyrus hallmarks, too, so it's not surprising iRacing inherited them - the only slightly dodgy thing (as with other Papy games, well any game really) is the tyre model, but there's
hope there too. Not much else comes close, and not much else (existing) could
supplement the physics in that regard.
However, iRacing can be intimidating to some, since the physics really need a steady hand sometimes. In that sense, a "less hardcore" physics mode might appeal to some, although that was trialed and aborted in GT5

. Personally, I feel GT5's physics are a great compromise. Never tried Forza. The GTRs and rFactors aren't a patch on iRacing, and they're not particularly friendly either, physics-wise. So, for me, the two "modes" might be iRacing's driving feel or GT5's driving feel, all underpinned by iRacing's physics engine(s).
These other games have more to offer than just their physics, though, which is more the point of this thread (i.e., rather than just blindly put
everything into a mixing pot, put specific parts of specific games into different pots and offer them up as different "courses" in the meal - particular features in our "perfect" game. Mixing together too many things that are so disparate leaves you with something that can only be "average".)
Certain customisation aspects from games such as LFS, Forza, (parts of) NFS and Sega GT (carrosserie!) would all be great additions, for instance. The latest LFS patch allows open course configurations, too, which is a surprisingly fun thing. I can't wait to see what layouts appear online. Presumably, GT5's "planned" free-roam mode would be similar. This has been my dream for so long now it's practically unreal.
Anyway, there are definitely some interesting suggestions so far, it makes me wish I'd played more sims / racing games over the years for better inspiration! I definitely believe that, no matter how sim-y a game is, there's no reason not to have proper
game aspects to it - that'll never happen for iRacing, incidentally, so let's dream on!