Don’t know about the best but probably the oldest and certainly one of the most useful sites for flow cytometry IMO is the listserve maintained by Purdue University.

Need protocols, help trouble shooting, seek expert input from seasoned, even famous, flow cytometrists? Then the Purdue flow cytometry listserve is the place to be.

To receive notifications, and post questions and comments, need to sign up with an email: Cytometry Info Page

They also have a searchable archive dating from July 1992 till date here: The Cytometry Archives