SIR is fantastic. Anwida is pretty good, but I couldn't get it to sound nearly as good as SIR for my use, and my version of Anwida had a bug that causes the box to crash after a minute if the plug is active without the file actually playing.
Room Machine is also great for some things.
They're all freeware.
My favorite reverb that I've bought is the ReverbX by Timeworks. But even with that I use SIR the most.
Are you spending money or looking for freebies? With freebies I'm not sure if you're going to find anything as good as Sonar's Sonitus reverb, except for maybe SIR. The freebies can give you some decent good sounding alternatives, though.
If spending money, there's quite a few choices available that can get you closer to hardware. I recently got a UAD-1 and purchased the add-on EMT Plate 140. That reverb will probably be all I ever need.