A lot of good info above.
If you are buying your first muzzle loader for hunting, I think the most important thing is to look at the muzzleloader regulations for the state that you will be hunting in. Some states have minimum caliber requirements for certain game animals. Other states may or may not allow inline muzzleloaders, 209 primers, pellet powder, or scopes.
My home state of Montana does not have a separate muzzleloader season, but they do have "weapons restricted areas" where muzzleloaders are legal, but only with lead bullets, and no sabots.
I started muzzleloading hunting years ago when I lived in Colorado. It was a separate early season and the first year .45 cal was legal for deer and elk. I made a .45 cal Kentucky percussion and shot the largest mule deer that I have ever shot. A year or two later they changed the regulations and required .50 cal as minimum for elk. I then built a .54 cal Green River Hawken, but I never did shoot an elk with it. Right after I built the Hawken, I moved to Montana, and as they don't have a separate ML season, I kind of quit ML hunting. A few years ago I did buy a .50 cal scoped inline ML that uses 209 primers, so I guess I'm ready for anything.
Like someone else posted, the .50 cal is a good compromise. Maybe not as flat shooting as a .45 and not as big a bullet as a .54 or .58, but certainly flat enough and with enough power to cleanly kill a deer or elk at 200 yds.