You get what you pay for. Lyman Great Plains in .50, roundball shooter will kill deer and you can load it down for smaller game. With light loads it will not blow game into furbugers.
Stay away from cheap flinters as they often have a lot of problems if the lock is not strong and the touch hole out of place.
What he said.
You could throw us a bone here and tell us at least whether you are leaning towards a rifle or a smoothbore, period of time (if any) you are interested in, style, cost, etc.
Flintlocks, much more so than in-lines and percussion ignitions, depend on a careful execution of the lock geometry in the design and inlaying work. It takes a delicate balance to get solid, surefire strike across the face of the frizzen; while at the same time not pounding it so hard that the lock destroys the flint in a few shots. There have been some dismal flintlocks offerred in the recent past, and I'm sure there are some lemons out there amont the latest crop.
Flintlocks are on the upswing currently - and that is wonderful. I would say without hesitation that what is out there at this moment is the widest selection and best quality of mass produced components ever offerred.
I'm not sure if we have any on this forum, but it would take a full-time effort by an individual to survey the gunshops and gunshows to see first hand what is available. Your best bet is to narrow your options and then ask for opinions on specific arms from our list members here. Chances are someone has experiences to share (we're almost at 800 members!)
If you stick with T/C and Lyman you can't go far wrong, though any used firearm could have had an abusive or ignorant prior owner who harmed the gun. If in doubt, arrange to have it checked over by a gunsmith as part of the sales agreement.
If you decide on a kit, go for one that is 90% (or more) pre inletted and includes a barrel already mated to the breech-plug.
If you opt for a smoothbore, you might do well to choose one of the Brown Bess copies now available. As an 'entry-level' flinter they have a lot going for them. That big 'ol 1" wide flint usually gets sparks even if the frizzen isn't tempered fine and the lock strikes are 'iffy'. And, they're a blast to shoot (n'yuk, n'yuk). You can stuff in shot or roundball as the mood strikes you.