Okay, so I think my top 5 villains are all quite psychotic or HAVE BEEN at some point in their lives. I'm going to rank all 5 of them out of 10 of how evil I think they were during their darkest times.

1. Regina as the Evil Queen during her big bad rein: 8/10
I gave her 8 out of 10 because she was pretty evil, no kidding. She slaughtered villages, ripped out hearts, tore families apart and trapped people in her castle who had done nothing wrong. But now, she obviously isn't like that. The 8/10 was purely based on what she USED to be like.
2. Cora during the first part of season 2 as the main villain: 9/10
I gave Cora 9/10 because she was definitely evil. Her and the Regina I just described above, weren't so different. Cora slaughtered villages, killed innocent people, ripped out hearts etc. The main goal for her was getting to Storybrooke to see her daughter again, so we didn't see her killing much people (bringing some back to life, even :P) but she did kill poor Johanna (I laughed, sorry) and kidnapped Archie.
3. Rumpel (not Gold) in his dark one faze: 9/10
Him and Cora are pretty much the same in how evil they are. At their most evil points they both had a similar goal: seeing their children again. He killed innocent people, ripped out hearts, separated families just like Cora and Regina, so that's why I gave him 9/10. No kidding, he was psycho.
4. Zelena where she is now: 7/10
So far she doesn't seem as evil as most of the villains on the show. We haven't seen her tearing out hearts, or killing innocent people yet. She might, no doubt, but still we haven't seen that so far. She is a bit..well, weird, in the sense that she wants someone else's baby and will probably try to steal it eventually.
5. Peter Pan during his bad reputation in season 3: 10/10
This guy was more psychotic than anyone. Mainly because he didn't have a reason for stealing children from their homes, when he only wanted one specific boy. Maybe he was lonely and needed sidekicks but it's still a bit weird. He swapped bodies with someone just to stay young, which is also quite weird. He has the most evil stare, I actually got scared by it!

Note: I don't think Regina is evil anymore, obviously, unless she's around Zelena and goes all Evil Queen on her ass. I can't comment on Rumple in the present day because he isn't in my top 5. Only Rumple, the dark one, in FL.