In terms of
reliability/simplicity, the `61 Springfield, Zouaves, and Enfield rifled muskets are exceptional. Lyman Old Style Minnie, 60gr/2Fg, and shoot.
My First recommendation for entry (and then keeping forever)
My Second in sequence would be a Lyman Great Plains percussion (50 or 54cal). That will introduce you to the never-ending combination game of ball diameters, twist rates, powder granularity/amounts, patch materials/thicknesses, and lube types/amounts.
Definitely THIRD are the flintlocks -- not because they aren't the funnest thing since sliced bread, but because they combine EVERYthing above -- and ADD the new (and artful) variable of primitive ignition, flint types/sizes/mounting materials and frizzen geometries/surface hardness.
Stay w/ real black powder.
I say again....
Stay w/ real black powder.