I'm going to give my vote to the Titans. Rather than liking them as the series' antagonists, I find them despicable and am appalled by their nature.

Yami is not much a villain, but only he activated the seal.

They were just there...

I think Angela is the just a bit

I don't really see Yuno Gasai as a villain, though.
She's more anti-heroic. ^ ^

Katherine1517, I agree with that as well as Lucy.

There's actually more than one character here who I don't really consider a "villain", such as Lucy and Akito.

please, the titans MADE the show

^I do agree that they are crucial to the series, but the Titans, themselves, eh, I don't feel like explaining this. *sighs*

^ (so what you're trying to say is...) They give the characters enemies, but they're not good enemies, right?
btw, never watched attack on titan. That's where they're from, right?