If, by Best, when you are talking boxlocks, you mean most reliable, I guess Id start with a basic A&N, sans either ejectors, or a single trigger. As George Hoenig
says, If they arent there, they cant cause you problems. Make sure it is double under bolted, then add a third fastener. Disc set strikers are a must. Specify 3 proof. Either 12 or 20 gauge to make things easy.
There are Italian, English, and Spanish guns that were produced in this configuration. Any would likely hold up well in the field, and require little in the way of skilled maintenance. Sear angle and access to locks seems to be more of a sidelock issue.
Someone on this website who claims he owns and uses American guns because he is poor, isnt being honest. Actual poor guys who want a double should start looking at guns off the continent, not American guns. Double guns from Belgium, France, and Germany are often far better buys.



Best,
Ted