Hello,
I think we have to make a difference between, the best car and the car that has the most potential. In MOE, the drivers drive with nearly 100% of the potential of the car and the FZR is the one that has the best one BUT that only happens with elitist leagues...
The XRR has lesss potential but his potential is easyer to use than the FZR (that car is the only GTR to burn CT too) and the FXR has less potential than the XRR (well... even that isn't really true since the FXR is better in AS7 and SO4) but is really easyer and have a big tyre problem (the front wheels burn!!!)
Now the FWD GTRs: The UFR is clearly better in nearly (all?) all tracks and burns less tyres than XFR. The only little advantages for XFR is that it has a sequential gearbox (and I agree with this, having the choice between H gearbox and sequential gearbox is really cool) and that it is a bit more stable.
But some people continue to use XFR since it'd funnier for them than the UFR that is more like a pot of yoghurt...
I'll show some examples with a French Endurance league I'm admining that has 3 categories: GT500-GT350-GTlight