One thing that should be noted is that most of the American shotguns are made in Italy, Japan, Turkey or Hungary...that being the case you might as well buy a foreign label...
If the skies the limit then any English/Scottish bespoke maker will suffice--Holland & Holland, Westley Richards, James Purdey, BOSS, Churchill, David McKay Brown, Dickson & MacNaughton and most of the Italian houses as well...prices will begin in the $85,000 range for field grade guns...
Also don't forget the German gunmakers out of Ferlach like Peter Hofer or Hambrush! There are also some fine Belgium and French makers Lebeau Corally, Vernay Cheron and Spanish makers like AYA who make bespoke quality pieces...