I kinda lean towards the Echo and the Burning as being the best villains, actually, in terms of power. No one really understands them, and the Doctor couldn't fight them. They get inside you and then there's nothing you can do. (for reference, everyone who doesn't understand me which is basically everyone who's not me, the 'Echo' is what I call the creature from Midnight, and the 'Burning' is what I call the one from 42). I wish I could find some way to incorporate the words 'Burn with me,' into the Woman in Black item.
I'm glad I'm not the only one who thinks this about the Echo. It was the only creature that genuinely scared and at the same time intrigued me. Loved the paranoia it cause on the train and how even in the end, you never knew what it was. To me it really is one of the best villains the Doctor ever faced.
I can't say I have a favorite. I hate them all. They are extremely scary and freaky, especially the biggest of the: Dalek, Weeping Angels, Cybermen, Are you my Mummy, etc.
In all truth, the most epic are the Dalek for obvious reason. I hate the Cybermen. It's like they just wont die, neither would the Dalek, but really. The cybermen just piss me off. The Weeping angels are just plain old scary. I don't care if they try to kill you in the nicest way, they are just plain creepy and freaky and if i ever see them I will bolt like sonic.
I'm gonna go with the evil force (the name of which I could not seem to find at all, anywhere) in the episode "Midnight" where Donna is at some spa and the doctor takes a trip. They're going to the a sapphire waterfall but they encounter trouble, and have to stop. After several minutes, one of the passengers begins repeating everything people say, to the point where it says the words at the same time. Whatever possesses this passenger eventually gains control over the doctor and starts saying his words before him.
I find this excellently creepy because it is something not even the Doctor knew how to fight, and the only way it was defeated was by a flight hostess who pulled it out of the airlock with her.
On a side note, this episode also has the only appearance of the best character to ever appear in the series. Jethro, a teenage child of two of the passengers, is one of the only people who remain rational and all throughout it he is hilarious.
I'm going with two- the weeping angels and the silence. The weeping angels for obvious reasons (there is a church with a weeping angel on the roof RIGHT by my apartment and I have to walk by it most nights) and then the Silence because you can't do anything about them. You look away and simply forget. Though I thought the way they "defeated" them was simply genius. Because seriously, they freaked me the hell out.
To quote Wikipedia (faaaaail sorry) "Actor Matt Smith, who portrays the Eleventh Doctor, called these aliens 'the scariest monsters in the Show's history' and Karen Gillan, who portrays companion character Amy Pond, commented that the Silence could 'actually rival the Weeping Angels in terms of scariness.' "