I know there are plays shown during Universala Kongreso, for example. I'd like to know which one had the most viewers.
The most seen play would probably be Gerda Malaperis since it is available on Youtube, which gives it a large reach.
In terms of a live theatre plays, it is hard to know because it is difficult to dig up records of attendance. But maybe this will help:
A play written by Carmel Mallia in the international language Esperanto, entitled Najbaroj (Neighbours), won first prize in a drama competition held annually by the Universal Esperano Association. http://www.timesofmalta.com/articles/view/20120819/social/Esperanto-congress.433453