Personally I can jump into rFactor, Live for Speed, GTR, Richard Burns Rally, NetKar Pro, Nascar Racing Season 2003, Grand Prix 4 and (and whatever else isn't immediately coming to mind, as well as console games like F1 on the PS3 and of course Gran Turismo) and it makes no difference to me. They all have something to offer, and they all captivate my attention as a sim racer. For the most part my 'skills' are transferable between them as well.
One game will do something the other doesn't, whether it's weather, multiplayer, modding, content, great artificial intelligence, physics... whatever... but they all have something about them... so you'd be mad to just stick with one game because you think it's the most 'accurate'. If I had to single out one game then it'd be iRacing because of its laser scanned tracks, and the multiplayer service it offers. The tracks are especially important because they really do transform the dynamics of the driving - and the multiplayer service is a great mixture of the existing models. But, as I say, any of the games will do as far as I'm concerned.