Funny how much it depends on having a great person on the front lines and a corporate philosophy in line with taking care of your customers. Mesa is not a small builder but do great. A few years ago I would have said THD.
Since the thread is about customer service, I presume that's a separate question from quality product in the first place, and is a matter of how they are when you need help.
Bad Cat has been terrific when something went wrong, full (and free) factory rebuild of an amp that I bought used that wasn't 100% at the time.
Jim Seavall at Scumback is fantastic to deal with both to buy the right thing, then to fix problems if any - like a speaker in an amp that was dropped a few feet leading to a Scumnico which needed a new voice coil....