Contrary to some of the above posts, I'm actually convinced that Line6 has the most accurate modeling (both in terms of modeling process and in terms of component / system model accuracy)
I say this from personal experience playing some of the actual amps (please note, I don't use FRFR, cab simulation, IR etc. and run into SS Power amp into actual guitar cab). With this setup - it is unbelievable how accurate the tone AND feel is.
My experience is born out in many videos online as well. Here's a recent one - Kemper vs Fractal FM3 vs Helix vs Mooer GE 300 vs HeadRush: which is the best modeler of a SOLDANO? - YouTube
I'm actually not in favor of approach that NDSP Quad Cortex is employing - i.e. using neural nets etc. This to me, is the 'easy way out' - you can make a neural net do almost anything with the right neural net architecture.
Line6 (and Fractal) on the other hand do component modeling (by and large) which is physics based. In the future I do expect neural nets to become more prevalent either as the base approach or as augmentation to physics based modeling. However, I always believe that you should start with actual physics-based modeling as the foundation for a truly sound model (I believe Neural also has component modeling on the non-profiled models). Where possible, I prefer to strive to improve the physics based model as that reflects the true working of the amp before employing any other techniques to minimize the overall error.
Now, the fractal does give you a more 'polished sound' (courtesy the beautiful reverbs etc. on the Fractal units)...but that doesn't mean that it's a more accurate sound in terms of amp modeling. Everyone is really close to each other now.