I also think this is a personal preference. IMO I like a 4.5-14 and think it's about perfect on a rifle for 100 yards, however, I don't know that I'd want something that big of a muzzle loader. On my muzzle loader I run a 3-9x40 and for me it's the minimum I'd want. I shoot it on 9x at the range, and I leave it set on 5x most all of the time in the field, but if I have time, I zoom it into 9x quite often before taking a shot at a deer. If it's something where he is moving, running, etc, where I wouldn't have time to adjust the magnification, I can still get a shot off with it on 5x and that's what I usually do.

I hardly ever use anything less than 5x, but I have once or twice if something was super close.

I've got a buddy that likes a 6.5-20 and most of his shots are in the 100-150 yard range although that's too much for me on the low end. I've got other friends that use a 2-7 and it's plenty for them, so it's all a personal preference.