Not really a practical option as these are extremely rare, but if you happen to get lucky and find a SONY TAN 8550 V-FET amp, grab it as fast as you can! Good condition one will cost $1500-$2000, only challenge is getting replacement V-FET if you blow one. It's in a different league than most of the amps mentioned in this thread, and I've auditioned many of them back in the days.
If you are into tubes, somewhat less rare are the Quad 2 monoblocks. Make sure they have the original transformers, all the rest is easy to replace.
If you are into tubes, somewhat less rare are the Quad 2 monoblocks. Make sure they have the original transformers, all the rest is easy to replace.