My reply to another one of your threads:
limestone, sandstone, and granite are all the best imo. slate isn't a good choice. It's too flat and doesn't create any natural caves. The only cave systems you can make with slate are rather cut and paste triangular ones which are easily toppled. I think slate is a good accent rock for some aquariums with more types of rock, but it just looks awful in mbuna tanks to me.
Pretty much any plant you put in the tank will be annihilated. I don't think there are exceptions with mbuna. Lava rock is save too, but it's sharp, ugly, and relatively expensive (around here, anyway). Lace stone is also a pretty common choice, but once again, sharp, ugly, and relatively expensive around here.
here's a photo of the progress of my 50 gallon tank, I chose to use black granite for the stone work.