Just curious, what is the best you've owned?
To be fair let's divide the group into 2 classifications - open sights & scopes.
I've been burning black powder since the 1970s, and probably the most accurate open sight gun was a T/C .36 Seneca. I know that goes against the "bigger is better" theory for a round ball gun, but I could (and have) shot 3 shot scores of 30/3X with that rifle shooting offhand at 100 yards! Of course the sights were a lot clearer back then, seems like they all use those cheap sights that look fuzzy these days!
For scoped muzzleloaders I'd have to give #1 honors to a Remington 700 MLS (stainless). Consistant sub-MOA accuracy with a variety of bullets!
A close #2 would be a T/C Omega. One of the early ones, and it would keep them close to MOA or under with a few different loads. About 1.5 MOA with 150 gr charges!!!!!!!!!!

Think back and remember the "sweet" ones, then do like I do and beat yourself up mentally for letting go of them. [:@]
The Remington was a pain to disassemble to clean for me, and the Omega was lost to a weak moment when a newcomer in muzzleloadingasked me where he could geta good used rifle.
The Seneca went because the .45 barrel I bought for it made the balance terrible, and I couldn't use it for deer hunting. Accuracy with the .45 barrel was terrible - not the barrel quality, butthe balance was just too bad to be able to hit anything offhand. I wanted a deer rifle, so I traded the Seneca on a Hawken.