Ans: There is a fluid known as "Pitch" which has the highest viscosity among all the fluids known to humanity. [Until 2023]
The "Pitch" is also known as bitumen, asphalt, or tar.
The viscosity of the Pitch is 230,000,000,000 cP (centipoise)
(or)
The viscosity of the Pitch is 230,000,000 Pa.s
It takes decades to form a drop of Pitch. An experiment is running to observe the fall of the drop since 1927 and it is known as the Pitch-drop experiment.
Link to the experiment.