it doesn't matter if it sounds natural or not. 97% of the listeners won't be able to tell naman kung amp sims ang gamit or real amps.
this is what I always say hehe
but to answer the query for the sake... I've read and heard that Amplitube 3 is the best at the moment. For rack type na digital, Eleven's got great reviews.
I'm happy with GR even the old versions. I have yet to try Logic Studio 9's amps etc...