I normally find this kind of question impossible to answer, but, when it comes to build quality, I think there is a company that has a record to sustain the evidence that it is a top contender, and that is Western Electric. There are many of its amps that are 80 to 90 years old that are still running and delivering world-class sound. I've heard terrific modern builds using old Western Electric parts, including 70 year old capacitors and resistors. Some other brands may, in the future prove to be that good, but, for now, I find it hard to beat Western Electric.
Highest BUILD quality tube amps?
Not so much with sound...lots of ways to fine tune thru tube rolling, etc. I'm just curious which tube amps are built to last for many years with the least amount of repairs. I know Mcintosh comes to mind for longevity. I was snooping around on the web and Air Tight seems to be built like a tank and I've never read any poor repair histories. Luxman same thing. Any others come to mind?