I agree that free access is important, but that doesn't count for much, if you don't put any marketing in. Unless the film is so controversial and well covered by international press, that everybody looks it up.
Therefore the ideal avenue would probably be a Netflix release (The Social Dilemma had like 125 mio. views). Or a theatric release, if the local distributors put in some marketing budget.
It also depends where the budget comes from. The film funding I am applying for aims at a theatrical release for example. And since it's German taxpayer money they prohibit a Netflix release for 120 days after the theatrical release and a TV/free release for 12 month.
If you think back at the big society-changing documentaries, none of them was a free release (ok, there was no YouTube when "An Inconvenient Truth" came out). I might be wrong – if you know any examples of the contrary please let me know.