Define best. Most innovative, precise, does beautiful work, ties the most difficult flies, most prolific, etc.? Genre of flies? Salmon, Trout, Salmon/steelhead, Warm water? The question is a bit broad and subjective. AK Best does some amazing work. Ed Engle, Lefty Kreh, Clouser, Gapen, Craven, Popovichs, and on and on. Bob Lemay ties some precise stuff that works. St Apte? Ron Alcott?
In my opinion Sid Glasso was one of the very best.