For a flinter, I would lean towards a Lyman Great Plains Rifle for a mass produced gun. Currently, I have a TC Hawken FL and percussion. I have fondled and shot the Lyman, and think its a better gun. The parts in the lock are better quality.
I am not current with who makes great custom traditional guns. I lost interest in sidelocks about 15 years ago.
I second the Lyman Great Plains suggestion. The one I have has shot 2" groups at 100 yards using the iron sights it came with. This rifle will have the slow twist intended for round ball; 1-60".
The load my rifle prefers is a .495" Hornady swaged ball over 85grs. (volume) of FFg Goex using an .018" patch lubed with mink oil.
Here's a link to honest traditional rifles, Track of the Wolf - Flintlock rifles
You will find quality rifles in traditional patterns not some contrived, good for sales replica. You can not compare a hand built flint lock to a commercial, it's like comparing Shiloh Sharps to the knock of Sharps!!
Take a look around at this web site. http://www.longrifles-pr.com/
There are Flint lock guns and then there are Flintlock guns. You get what you pay for. Both of the web sites posted here are good and pretty much only sell Good Stuff.
Here is another web site. These guns are awesome. I have used their parts and stocks to build guns with and even some of their kits and quite frankly they are top shelf guns. You pay for it too but the quality can't be beat. Jim Chambers Flintlocks