Never going to get everyone to agree, but here's my opinion:
GTR - I love it - the combination of audio, wheel and visual feedback tell me exactly what the car is doing at any point in time. Took a bit of fiddling to get the wheel settings right, wasn't great until I got that sorted. Multiplayer is quite bad, but it is workable, once you know all the problems.
NR2003 (road racing mods) - I hate it, however the online community is HUGE and, mostly, friendly - I race this a lot in a local (SA) league where we are regularly racing in fields of 20 or more drivers, which is just awesome. The drivers tend to be the most considderate, probably because of the truly terrible collision physics in online play.
rFactor and GTL - don't really want to commit until they get released, but looking very good so far.
LFS - only played the demo and had the licence for a day now, but it looks very good, once the wheel is set up properly and the insane oversteer is dialled out with the setups - as was mentioned earlier there is huge potential here, and it's only a alpha release, so I expect this to get better and better.
As far as multiplater goes, performance-wise, in NR2003 I can host 15 people (router limits the number of ports I can forward) in GTR 9 (the 10th starts showing signs of warping) and LFS 6!!! Slightly contrary to what is being said, although the LFS limit seems to be atifically imposed to keep the quality high. Multiplayer quality-wise LFS seems to be the best, closely followed by NR2003 with GTR trailling quite a way behind (but still better than any previous ISI based sim)
Bottom line is, there is a place for all the sims, each has their good points and bad points, and each seems to be aiming for a different goal, so the only real solution is to get them all
Chris