The most famous, or rather infamous, Crowley is Edward Alexander Crowley, better known as Aleister, dubbed by the Sunday Express "The Wickedest Man in the World".
I'm not going to go into great detail about him as there is a wealth of information available, but it would be excessive to ignore his existence. A good starting place for finding more about him is the his Wikipedia page contains a lot information, references and links. Mary Crowley's history of the Crowley family included elsewhere on this site also dedicates a chapter to him.
The two articles I have on him that aren't generally available are a brief column about his time living on Bateman Street in Cambridge and an article from the Tonbridge School magazine The Tonbridgian.