Adaptiverb really is like nothing else I've heard.
There's some kind of crazy AI Zynaptiq use that removes clashing notes left over from previous chords, notes, etc. This is is why it has this pristine tail that sounds like nothing else... Unfortunately it's pricy and a bit heavy on CPU but well worth it.
Supermassive took some time to grow on me. It wasn't until I started flipping through some free 3rd party presets did I realize it's capable of some pretty weird but useful stuff. Then again Valhalla's pretty great at that... Ubermod, VH Delay, even Space Modulator all do things way outside their description...
Overall though order of preference for me is Adaptiverb, Blackhole, Raum, Supermassive.
(Speaking of which.. I didn't mention Raum. It spent the 1st year or more installed and never touched. I actually use it all the time now. It's really great at long icy tails and very resource friendly. Sure it's not Adaptiverb but it's surprisingly much better than I thought when NI gave it away as a promo...)