Any standard 3x9 should work well for you. Sight in 3" high at 100 & you be around 3" low at 200 yds. 300 yds is a stretch for most muzzleloaders & shooters. I shot a deer last year at 210 & the lack of kenetic energy was noticeable.
I bought a muzzy last year at the end of season. After doing some research I decided on the Leupold vx freedom in the 1.5-4 x 20. I have yet to buy it but that will be what i put on it. I wanted to stay with low magnification and thought it was funny when you do a search for muzzle loader scopes almost all are in the 3-9 or more power.
I didn't really spend much time in choosing what scope I want to be honest with you. Yeah, sure... I did buy some pretty cheap ones that were pretty bad. Didn't take me long to understand where are the right places to look for better equipment. Now before buying anything, I check in on some hunting scopes, many of my close friends showed me where to look and how to properly check my equipment. A couple of months ago I decided to get my UTG 3-9X32 1 scope. Boy am I happy with this scope. Unlike my other scopes, this one doesn't get foggy and moist at all.
I got a TC Omega 12 yrs ago to hunt MZ with a scope & get some distance out of a frontstuffer.
At the time there were only a few scopes made for MZ. I saw a buddy use a basic 3x9 Leup & it blew up after a while. NOT meant for MZ use....
I got the Nikon 3x9x40 MZ & the glass is real good & clear ,but I hate the BDC dots. Granted they are spot on & you just have to put the pie plate over the heart,but you also have to have it set on 9 power for BDC to be acurate....& I rarely walk around with something set at 9. Trade off I guess.
Just following & wondering what has improved as much as range finders recently.
2x7x35 Burris FFII with the Ballistic Plex. Like Hank I am not a big fan of the BP, but I have to say it works on my TC Prohunter. With the Ballistic plex the crosshairs are dead on ( naturally) and the second hash down lines up perfectly at 200 yards.
That scope was on a slug gun for 5 or 6 years, then it graduated to the muzzleloader back in 2008. It has never missed a beat, has taken a beating and stays true.