Didn't think much of Amplitube. Tried Bias FX recently (demo/trial), didn't think much of that either. GR is OK to pretty good, in particular I like all the fx and it has some fun high gain stuff. For realism, I'd go with S-Gear every time; it's not a 'turn everything to 11' kind of plugin but it feels more real than any other plugin I've tried; it's not trying too hard to be 'dynamic' which is where I feel a lot of emulations go astray. It's not trying to emulate or model anything else, it's its own thing. Isn't it an ex-Marshall guy who produced it? Anyway, I'd use GR, TH2|3 (plugins) or the GSP1101, THR10 (modelling h/w) if I wanted to go to 11 or beyond.
I've tried pretty much everything there is and will continue to do. Some I really don't like at all (Amplifikation One, Thermionik), some is simply atrocious ('Hardcore' - b'wahahaahaa), much is rather 'meh' (e.g. Vandal), but there is some really good free stuff out there too (like Poulin's amps and cabs).
The point I'm slowly ambling towards is that pretty much all of it is free to try so do so and figure out what you like, what works best for you.