From La Sturmo de Johana Arimo to the animated short Cxapeloj, there are actually quite a few Esperanto movies out there. Which is the most well-known / most watched in Esperantujo? Would that be Gerda Malaperis? (Simply because it's the only one I have seen so far, I think.) I guess criteria could be YouTube views or public displays at congresses where an estimated number of viewers is known.
That would certainly be Incubus (1966) starring William Shatner. No Esperanto movie has reached anywhere near its fame. After this, I would venture that either the films of Christopher Mihm like Atako de la lunaj zombioj (although some would disqualify this, since it's an English movie dubbed into Esperanto) or Gerda Malaperis. Unfortunately sales statistics for none of these films are public, so past Incubus, it is very difficult to guess what the second most popular Esperanto film would be.