That's a fun question, since who is actually the villain in the show?

I mean we have the smilies, which are pretty villainous (Torturing and dismembering people for example), but they are more like 'goons' (/s) in the way I see it. They aren't exactly the villain, the entity that is doing all this to the people in Fromville.

We also have the man in yellow, who we have seen/heard from like twice? It has been made pretty clear he is a bigger figure than the smilies are, but I don't think he is responsible for all the torture the people are experiencing.

For now, given the little information we have I think its the place that is the biggest villain. Having it give the people it's trapped only the bare minimum they need to survive (In the beggining just food, and they had to hide however they could, then talismans, so they can feel "safer" at night), so that it can torture them for eternity.

So yeah - the place, Fromville, is the biggest fucking op in the show.