The BEST black powder? Boy, there is a question that will stir up a bunch of discussion. But, I think the
best powder is getting to be whatever brand you can get. It does no good to tell you that brand X is the best powder in the world if you can't buy brand X in any store. I think that you are more likely to find Goex in stores. it is an American made powder. In fact, it is the
only American made black powder. It is also the least expensive of the available black powders. Goex makes and sells another black powder under another name but it is still Goex. If you mail order, you will have to pay an hefty hazmat fee in addition to the price for the powder. The foreign made black powders such as Swiss are going to be considerably more expensive no matter where you get it. So, I'd say that 3f Goex is going to be the best black powder. It's what I shoot in all of my guns from .36 to .54 and I have no complaints about it at all.
If you can't find black powder locally and find yourself having to use a substitute (works only in caplocks), there are a number of choices available to you. I have tried Triple 7 and Pyrodex. Both work in a caplock but are nearly worthless in a flintlock. They are pretty darned expensive. Some folks say that they are more corrosive than black powder. I can't say since I haven't used any substitute powder in many years. But, if you have a caplock and the substitute powder is all you can find, it is better than leaving your muzzleloader sitting in the back of the closet. The best thing is to join a muzzleloading club. Not only do you have someone to shoot with but most clubs take orders for powder periodically and order in large 25 to 50 pound quantities and the hazmat fee is spread over the whole shipment making the cost per pound much more affordable.
Oh, and if you don't have as copy of this
http://www.blackpowderrifleaccuracy.com/ order it, read it carefully and follow it exactly for
amazing accuracy from your new rifle and any other muzzleloading rifles that you may own. Only 20 bucks for a
lifetime of muzzleloading accuracy? You can't beat that with a stick.