Below is a list of fastest rotating black holes measured in the dimensionless rotational parameter a.
Methodologies for spin measurements Accreting black hole spins can be measured by observing the X-ray spectrum of the black hole and identifying spectral features caused by certain characteristics of the accretion disc. This can be used to determine the innermost stable circular orbit and hence the spin. Another method measures the radiative efficiency of the accretion disc.
List
Prograde rotation
See also List of most massive black holes Orders of magnitude (speed)
References