I would have said S-Gear 2 right off the bat if Paul hadn’t asked specifically about free sim plugs in his OP… they’re really good for sure! And because it’s not freeware, you get lots more variation and a lot of included “presets” with particular combos of sounds, and those are always a great place to start, just cycle through them and see what strikes your fancy, then you can always get “under the hood” and start tweaking this or that parameter and just experiment. The other for-money amp sim that I go to just as much is Amplitube (I have v 3 and 4).
But for no-kiddin’ zero dollars, the Le Pou sims are hard to beat.
NIck Crow labs are also noteworthy. I pair their 6505 with Ignite Amps NadIR and Kalthallen freeware impulses, some of the best lead sounds in modelling IMO.
I agree with this - Amplitube never really sounds good until you find its right place in the mix - then it does the job proper. You just have to handle it when recording - knowing all will be well.