Audion Black Shadow mono pair. Right behind that, the Sophia 845 mono pair. Both are SET. I make it a point to seek better 845 amps, having heard and spent time with every alternative discussed here, plus some that aren't mentioned. Nothing so far has matched the speed, tonal integrity, neutrality and dynamic clarity of the Audion monoblocks, with the Sophia approaching closest.
The new Shuguang Psvane 845 PSET monoblocks look promising but haven't heard them yet on speakers sufficient for evaluating them seriously.
Phil