Last time Scoots did companions, so this time I'm doing villains. And to make this more varried choose something other than Daleks and Cybermen because we all love those. My top three in no order are The Master, The Headless Monks, and The Silence.

A threeway tie between Cybermen, Weeping Angels, and Silents. (The name Silence is the name of the religious order, not the species)

Cybermen (Particularly the early versions, the original voices, ok they might sound odd but they were inhuman. The way they argue that having your flesh and emotions removed are for the best)
The Destroyer (From Battlefield, a wonderful costume and a truly scary creature)
The Master (Most versions each in their own way, all are merciless but some will just randomly murder you just because they feel like it)

Missy. Specifically Missy, not the other versions of the Master.
After all, if we can have our favorite Doctor, we can have our favorite version of the Master, right?
So, if you say that your favorite villain is the Master, might I ask, which version?

4838769 Here's mine right after Prime Minister Master.

4838865 And my preferred Master Variant.

Oh, Clara Clara Clara! You know, I should shoot you in a jealous rage!

Well i would go with the master the derlacs and the silence

And of course i'd have to go with the first version of the master i saw when the doctor after saving Dona's father had to regenerate