Caliber? Ignition?
If you get something that can handle conicals (1:28" twist or faster), even a .45 cal is sufficient if it will take 100+ grains of powder.
For example, a 200 grain Lee REAL with 120 grains of FFg will do 2000 FPS, plenty for deer at under 100 yards. Never had one walk more than a few paces...
Now if you are restricted to traditional ignition (flintlock) and ball, only a .50 caliber would be recommended.
But if you can use modern rifle (shotgun primers etc.), nearly everything out there is more than adequate, providing you can put the bullet where it counts.
Sights are where it it at for accuracy. Even in my crappy .45 caliber, I was able to keep groups under 2" with the REAL at Mach 2. Yeah, I used a bit more than 120 grains ;)...closer to 2300 FPS
. Has to go up to wheelweights as leading was a problem.