Viper Racing is kind of limited compared to games of these days, though, and it has a terrible interface/menu system (though the paint editor was a fairly powerful system). I always found the way it modeled downforce to be weird, as well.
I personally prefer Sports Car GT for old school sim racing.
The default value of car physics really lack of testing. I think that what make the car feel lighter. I usually made the car 60% heavier on my mod.
Funny thing is, Viper racing use the same downforce value as GT2. Both game also has weird behaviour when we change the car weight to very light. a 10kg feel like a baloon on viper, on GT2 and GT4.
We can't change Sports Car GT tire behaviour. In Viper Racing, we can make the tire behave like Sports Car GT, like GPL, like NFS or even like GT4

. If you have time to tweak it off course

.
With right amount of testing, I think tire behaviour should be able to reach Nascar Racing 2003 level, since both share the same game engine. The difference is Nascar Racing use real life data and rigorously tested with real team mechanic, and Viper Racing don't.
Anyone know Racer? They say it has better realism than LFS. I forget what version, but LFS just recently change it's tire physics. This means tire physics in LFS still need improvement.
I think the best PC game simulation do not have to be the one with the most sophisticated engine (rFactor and GTR). But the one that use real life data and verified by real life engineer. Something like Nascar Racing 2003 or GPL. Unfortunately both game have fixed physics. So my best bet would be any game with Open Engine physics that allow user to tweak the tire data (like racer, Viper Racing, Nascar Heat, motorsport-sim, etc), that have been polished enough by either it's developer or it's fans.
From what I learn after browsing many PC sim website, it seems the one that use Pacejka tire simulation has the best realism. If your game do not support modification to tire data, then you have to trust the game developer for realism.