The rarest blood type in the world is called Rh-null or "Golden Blood." It is extraordinarily rare with fewer than 50 known individuals globally. Rh-null blood lacks all Rh antigens on red blood cells, making it highly valuable for rare medical transfusions but extremely difficult to find compatible donors for individuals who have it.

Among the more common blood types, AB-negative is the rarest of the eight main blood types, present in less than 1% of the population.

Additionally, the Bombay blood group (HH blood type) is also extremely rare, found in about 4 per million people worldwide, primarily in India.

In summary:

• Rarest blood type overall: Rh-null (Golden Blood) — fewer than 50 known cases worldwide.
• Among common blood types: AB-negative is the rarest.
• Another very rare type is the Bombay blood group.

This rarity makes Rh-null blood critical for transfusions in patients with very rare Rh antigen profiles but very scarce in availability.