[Cougra]

There really is no "Best substrate." Each has it's own advanages and disadvantages and uses for different tpyes of fish and tanks. If you explain more about the type of tank and aquatic animals you would like then we would be in a better position to help you decide on which type of substrate would be better for your particular tank.

BB tanks are good for SPS corals and tanks that require high water flow, but aren't very good for substrate dwelling animals that like to bury themselves or dig their own homes.

SSB is good for tanks that have inhabitants that like to live in the sand and for reef tanks that don't need as much current flow and can live in slightly more nutrient rich waters (button polyps, zoanthids, mushroom corals, leathers) , or for people who don't like the look of a bare bottom tank. They require vacuming and work to keep them clean.

Crushed coral, although a lot of people distain it, is good for predator tanks (with fish that like to dig to find food like triggers) or fish only tanks as a fairly inexpensive option to cover the substrate and can handle heavy water flow without clouding up the tank since it's too heavy to be lifted like sand would be. It too requires regular cleaning. However it's not good for fish that like to bury themselves at night as it's hard for them to dig up and it's large sizes make it easy for food to get trapped under it which, if not cleaned properly can lead to high nitrate levels in the tank.

DSB (Deep sand beds, again contriversial) are effective means of biological filtration, however they are very limited and can only handle very low bioloads in the tank. They also depend on living creatures to keep it viable which can be risky. It's not good for tanks that have substrate dwelling animals such as sand sifting seastars or fish that like to dig because they will remove all the creatures you want in the sandbed to keep it viable.

There are many other things that can be used as a substrate and each has it's place, and limitations.