Is solid hardwood the best in all cases?
Engineered floors (also known as Longstrip or Europlank flooring) are more stable than solid wood flooring, i.e. less likely to expand and contract with changes of temperature and humidity. A concrete slab will likely have higher moisture content than most plywood subfloors, making the more stable engineered floor a good choice. Another option is to seal with a moisture barrier and then trowel glue down solid hardwood.