I'm a snob when it comes to flintlocks...I own 2 custom built guns, both have Siler locks on them, they throw a very good spark and are reliable...
In mass produced guns, I hear good things about the T/C Hawken and the Lyman...Both seem to have good locks on them...
The next consideration is the barrel...My guns both have cut rifling, about .015 deep, the .54 has a 1-72 twist which is proper for a patched ball...You need to decide what projectile you will want to shoot...If deer hunting with a patched ball, I would suggest a .54...If you decide to shoot a sabot/jacketed pistol bullet, you need to go with a 1-28 twist gun...A 1-48 twist is a compromise twist that can shoot patched balls or conicals, like a Buffalo Bullet, Maxie, etc....If getting a gun to shoot sabots or conicals, I would go with a .50...
If you are able to examine the rifle before purchase, look at the touch hole...It should be centered over the pan and the bottom of the hole should be just above a line over the top of the pan...In other words, when you close the frizzen, it should cover the touch hole...If the touch hole is too low then powder runs into it and you get a fuse effect upon firing and it slows ignition....
Finally...Only use real black powder like Goex in a flintlock...I use FFF in both the barrel and the pan...
The ignition point of real bp is 480 degrees, where as the ignition point of something like Pyrodex is around 800 degrees...So your ignition is slower and unreliable....Most folks don't realize that when Pyrodex was introduced back in the mid 70s, it was developed so dealers don't have to store in a fireproof safe and could be shipped safely....The Marketing folks spun it like it was "New, Improved and Cleaner"....
Good Luck, hope this helps....