I use a Knight Wolverine. Walmart had some of the nice Thompson center Black Diamond rifles real cheap. My Wolverine was under $200 from Gander Mountain 12 years or so ago. Still shoots very well, as well as I can or better.
Once you get into the modern style closed ignition inline black powder rifles, any good name brand is a decent piece. By the way, my hunting buddy has an Omega and two stocks have cracked on it. Both replaced under warranty, but still, they cracked. I suggest that entry level black powder shooters buy an inexpensive model to start with. You might decide that black powder isn't your thing. Used top end black powder rifles generally don't tend to hold their value too well.