A lot of the language learning courses out there are free and of high quality, but there are also some that cost money. Which is the one that is used the most? I'd like to ask about the "best" resource, but that's a highly subjective word, so I'd rather ask about the most-used one.
It partly depends what kind of course you intend - one for individual self-study, for correspondence tutoring, or for group / classroom use; one using a given first language with maybe, translation exercises, or one wholly in the target language...
In the first category, for English speakers, the following has been quite popular: Teach Yourself Esperanto. In the second, , for English speakers: Elementary,..! In the third, using only the target language: Kunvojaĝu. And for advanced learners: Vojaĝo en Esperanto-Lando.
For groups of beginners, this one has been a best seller in recent years, and has been translated for use from a variety of first languages: Esperanto by Direct Method.
Like your subjective question, this is a subjective answer - there will be other equally valid replies!