I've looked into this several times before and it's actually kind of hard to get a good answer. The best we used to have is SuperData's charts (I think that was the name, maybe something similar) but I don't think those are around anymore.

The website mmo-population is NOT accurate in anyway and can't be used as a reference because of their methodology is junk (only gets a rough estimate on western games which is wrong in the first place, but also vastly underestimates games like DFO.)

I imagine it is likely DFO, but it might be WoW (or maybe even more likely some mobile game I've never heard of if those count). Very difficult to resolve properly anyways.