I own many amp sims and they are all very good, but S-Gear gets the call 95% of the time.
It sounds great with both a real guitar or a sampled guitar. For whatever reason, it always sounds great in my mixes.
It has easily been the best buy I have ever made on music software.
Guitar tone is a highly subjective topic and others may disagree.
S-Gear also doubles as a great effects plugin.