What is the fastest enzyme?

Carbonic Anhydrase is the fastest enzyme. One molecule of carbonic anhydrase is capable of hydrating 106 CO2 molecules per second, which is about several times faster than most other reactions that occur in the absence of this enzyme. 

Carbonic Anhydrase is an enzyme found in pancreatic cells, red blood cells, and gastric mucosa to name a few. It is involved in catalyzing a reversible reaction between water and carbon dioxide, resulting in the generation of carbonic acid. It also plays a role in catalyzing the production of hydrochloric acid by the stomach.