For side hammer guns, black powder is my first choice, FF or FFF depending on what you like.
In-lines, any of the black powder subs will do the trick. Pellets are the easiest but harder to dial in a load. Pyrodex or T-7 will be the easiest to find. Shockeys Gold, White Hots, BlackHorn 209 are usually the hardest to find.
Loose powder will allow for smaller adjustments for better accuracy.
A standard load that shoots pretty good in all .50's is 80 grain powder, 250-300 grain bullet.
And of course there is the evil smokeless muzzle loader which shoot smokeless powder.
I have all 3 types, side hammers, modern in-lines and smokeless. So far Ive killed deer this year with 2 out of 3 of them. h.h.