The guitar scientists at Ignite Amps have built a few free sims that sound terrific.
Anyone writing heavier music should definitely check out The Emissary.
It’s easy for really distorted guitars to sound flat, but The Emissary breathes plenty of life into recordings.
Make sure you download the newest version: Emissary 2.0.
With the 2.0 update, the Emissary has gotten a complete tech overhaul. Without getting into the nitty gritty details, it now sounds even livelier than it did before.
The tube saturation has also been improved, and you can save and share presets you create.