Jbar did a good job explaining twist. Slow twist (1:60, 1:72) for patched round balls, fast twist (1:48, 1:32, 1:28) for conicals, sabots, etc.
Also, with a patched round ball, understand that if you're shooting a .50cal gun, you'll probably buy a .490 or .495 cal ball. Then, a patch, which may be .18 or .12, etc. thick. The thicker the patch and the closer to .50cal you go on the ball, the tighter the fit in the barrel. You may have trouble ramming the ball/patch home. But, sometimes a tighter fit may result in better accuracy. It all depends on your rifle and what it likes. Folks experiment all the time with bigger balls or smaller balls, thicker patches or thinner patches, to come up with the best accuracy in their gun.
As for which brands to avoid, I guess it all depends on what you want a muzzleloader for. Rifle? Pistol? Hunting? Target? Generally, like anything else, you get what you pay for. The more expensive the gun, typically the better quality (not always, of course). Lyman, T/C, Pedersoli, Navy Arms, they all make good quality guns. CVA is a lower quality gun in some respects, but many folks have had good luck with them.
Check out the guys on this board for a wealth of information on muzzleloaders:
http://www.muzzleloadingforum.com/ubbthreads/ubbthreads.php