I use the Black Tahitian Moon sand.
I've also used the black/white mix (I think it's called Indo-Pacific black) but there are large pieces of black rock/pebbles in it that make siphoning the top layer a real issue because the pebbles get stuck in the siphon tube.
Tom
p.s. Whatever you do, stay away from Estes marine sand. It is powder-like, floats, and is a real mess to deal with.