What is the best black powder substitute?

There is obviously no single answer to this question, as it depends for what purpose you are wishing to load, and what kind of firearm. For what it is worth, my own favourite substitute is American Pioneer Powder. It substitutes well, volume-for-volume, for black powder in all applications I have tried, except for flintlocks, where it is probably best to stick with real black. APP works well, however, in cartridges, and in muzzle-loading rifles and shotguns, and in percussion revolvers. It makes a nice boom and a big cloud of smoke, pretty much just like black powder (ok, the smoke is a slightly different colour, if you're that observant). And best of all, it cleans up very easily - more easily than most smokeless powders, and a lot easier than real black. Generally a couple of quick patches through the bore with a solvent such as Ballistol or Gunzilla is all that is needed, although the actual ignition area requires a sightly more thorough cleaning. With cartidges, you do need to drop your used brass into some soapy water or similar as soon as possible after shooting, especially on humid days, or the brass will go green before you have a chance to clean it. So American Pioneer is my first recommendation, at least for fun and for re-enactment etc. For hunting you might want to look in other directions - although I'm sure you could hunt with APP. Triple Seven is popular with hunters, especially those using modern type muzzleloaders, as it is reliable and a bit more powerful than real black.