There are several fields here joined together. It may look like a field, which in a sense it is, but it's not a perfect rectangle.
The market is fragmented
+ academics influencing business (often from universities with very impressive reputations, but again there are those who see it as a branch of psychology and others who focus more on applying it to the world of work)
+ purer play OP experts like Kaisen
+ top strategy houses including McKinsey, Boston and others
+ other MC's (OliverWyman is a very reasonable example)
TP are more known for areas like benchmarking performance, reward policy in areas of financial services that employ vast numbers of employees.
So there isnt a leader - but I guess it would still be very tough to ignore McKinsey in terms of leading organizations.
It is a vast area of consulting, and one that really does offer a gateway in from academia so dont be easily put off. Oliver Wyman are certainly top contenders.