Best experimental directors?

Is Tarkovsky considered Experimental?

in addition to the ones you've mentioned:

6. peter greenaway
7. the brothers quay
8. don hertzfeldt
9. harmony korine

honorable mention: lars von trier. he doesn't stray from traditional narratives, but he's reinvented his process so many times (with a lot of hits and misses along the way) that i find something admirable about his work.

Stan Brakhage
Matthew Barney

Disco sucks, kill all hippies. Pretty vacant, eh? Subvert normality. Signing off.

Kenneth Anger
Andy Warhol
Godfrey Reggio

I would say that Tarkovsy is experimental. He may not be anything too crazy now, because his style has influenced so many people, but I would say that back then he was rather earth-shattering.

Stan Brakhage
E. Elias Merhige
Kenneth Anger
Isidore Isou
Guy Debord
Jean Cocteau
Jonas Mekas
Marie Menken
Sidney Peterson
James Broughton
James Sibley Watson
Dimitri Kirsanoff
David Lynch
Man Ray
Marcel Duchamp
Maya Deren
Willard Maas
Andy Warhol
Ron Fricke
Godfrey Reggio
Terrence Malick

Abyssus Abyssum Invocat

No, generally he's admitted as film theorist who was mainly captivated with metaphysical themes but at this time his method of approach was dialectical. We can't say for each director who disclaims conventionalism that he's experimental.

I would say yes. His use of the long take was revolutionary back when Rublev was released. Tarkovsky and other Russian directors (German, Konchalovsky, Mikhalkov, Klimov, Sheptiko...) were against Soviet Montage, and wanted to extend cinematic moments beyond mere seconds, or fractions of seconds.