Rich's description of Jimmy Thach's contribution is right on. Only thing to add is that his "beam defense" tactic formed the basis of "loose deuce" in the jet age, which held (holds) up even in an air-air missile environment. Thach's doctrine essentially supplanted Molders' finger four, which some air forces still use. The USAF retained finger four throughout Vietnam and sometimes got hammered as a result.
Anyway, the enduring nature of Boelcke and Thach's contributions easily rates them No. 1 and No. 2, respectively.
I considered Marseille as No. 5 but he and Hartmann essentially did the same thing--led staffels while building huge scores. Hartmann eventually was a nominal group CO but he remained much more a shooter than a leader.
Marseille would be waaaay up there on the list of Fighter Pilots of the Millennium, especially since we now know that over 80% of his claims were valid. Russell Guest in Australia is tracking that subject.
As for Nos 6-10--hoo boy! I'll toss out five names that occur to me and the forum can thrash 'em over. Remember: we're looking for influential leaders & innovators.
Douglas Bader (RAF WW II)
Robin Olds (USAF WW II & VN)
Keith Park (BrisFits WW I, RAF Fighter Command WW II)
MTS Pattle (S Africa WW II)
Alexander Pokryshkin (USSR WW II)