The rarest blood type in the world is called Rhnull, also known as "golden blood." It lacks all Rh antigens on red blood cells, making it extremely rare, with fewer than 50 known cases worldwide and an estimated occurrence of about 1 in 6 million people. This blood type is highly valuable for transfusions involving rare Rh blood types but is very difficult to find donors for because people with Rhnull blood can only receive Rhnull blood themselves.

Other notably rare blood types include AB−, which is the rarest of the eight major blood types, and the Bombay blood group (HH blood type), which is extremely rare primarily found in some populations in India but still far more common compared to Rhnull.

Additionally, a recently discovered rare blood type called Gwada-negative from Guadeloupe has been identified, though it is currently known from only one individual and represents a new blood group system.

In summary, Rhnull ("golden blood") holds the distinction as the rarest blood type globally due to its absence of Rh antigens and the extremely limited number of known individuals possessing it.