I think it comes down to IRs a lot. There are so many very good amp sims, either free or not, but the final tone is so much driven by the quality of the IRs you use, that you can get really good results with any amp, as long as you have good IRs.
That being said, I really like the following amp sims:
- Thermionik (Framus Cobra, ENGL Savage and Mesa 3CH Rectifier)
- LePou (ENGL amp and their "Legion")
- Nick Crow (5150 and 6505 sims)
- Ignite Emissary (based on their own custom design Emissary tube amp)
The IRs I like to use:
- 3 Sigma Audio
- Rosen Digital
- Wall of Sound
- Valhall IR (a recent discovery that I really, really like. My go to IR since a couple of days).