As long as whatever rock you use is inert, wouldn't the idea of "the best" be a matter of personal opinion? It would also be influenced by the particular application. Some types of rock (and colors) would work better than others if you were trying to create a particular look or to match a certain biotope.
Do you have something in mind already that might look good? If so, why not test it. Get some muriatic acid from the paint supply section of your local hardware store and pour a bit on. If it bubbles, then move on to the next selection.
Last edited by sparrow; 03-31-2006 at 12:54 PM.