I own pretty much all of the modelers for some reason ; and some traditional amps, too. Too much disposable income and a tendency toward habitual gear collection, I suppose. First world problems and all that.
Anyway here's my personal awesomeness hierarchy for general amp modeling technology based on my preferences (decidedly heavy sounds, starting at classic rock up to modern metal with an emphasis on 80s hair and thrash sounds):
AFX3 > AFX2 > Atomic > Helix
Helix is a good unit and has a lot of value to offer (it is
great compared to other Line 6 products). But
tonally both the II and III iteration of the AxeFx are pretty far beyond it in terms of feel and accuracy to IRL tube tones that I go for.
It is popular on boards like TGP etc to quell flame wars to say "everything is so good now that it doesn't really matter or; it is a matter of taste/opinion" and other platitudes to keep the peace but honestly, it ifeels like a substantial unbridgeable gap if you are picky and have tastes/experiences similar to mine.
In particular in Helix the feel is definitely off, you know you are playing a processor when you use it, it just doesn't feel tubey to me. Even the amplifire smokes the Helix there. This is the biggest gap.
Soundwise, the Helix frequency response in the high end of distorted tones is 'off' it has too much of some frequencies and not enough of others relative to what my expectations are for the modeled amps and other processors (FAS, Kemper, Atomic, et al). Once you notice/hear this it is impossible to dial the thing in without it grating on you somewhat, IMHO. You can EQ and pick different IRs, etc; but it feels like you are fighting the unit for that last few % of your tone wheras the FAS units just deliver it directly from the start.
In terms of power amp modeling
nothing compares to Fractal. The III just took a BIG leap here too. I cannot wait to replace the Atomic and Line 6 with the FM3 when it drops.