wouldn't say GTR is more realistic.. or R factor.. played all 3.. would still say GT4 is a very well mapped out physics.. Live for speed however, would probably take the cake..
problem with the above - GTR and R factor, is although they are meant to be realistic, they are still somewhat arcadey in their feel.. i've just cranked GT4 out again (got a HD 32" LCD - really makes a difference)..
maybe if I had a steering wheel for the PC games it would make a difference.. live for speed would have to be my pick.. great fun..