Which type of cloud most often produces severe thunderstorm and hail storms?

Cumulonimbus
Cumulonimbus, from the Latin cumulus (“heap”) and nimbus (“rainstorm”, “storm cloud”), is a dense towering vertical cloud associated with thunderstorms and atmospheric instability, forming from water vapour carried by powerful upward air currents. Cumulonimbus can form alone, in clusters, or along cold front squall lines. These clouds are capable of producing lightning and other dangerous severe weather, such as tornadoes.