Nightmare Moon: The Dark Threshold (She put the Mane 6 in high pressure situations that secretly tested them)
 
Discord: The Force of Disorder (Before his reform, he seemed to have no other purpose than infecting the world with chaos)
 
Queen Chrysalis: The Ambitious Succubus (She may have the simple goal of feeding her swarm, but the way she goes about it is as ambitious as a vampire seeking the blood of a ruler)
 
King Sombra: The Evil Presence (Most of Sombra’s threat factor comes from the things that he set up near the end of his life becoming dangerous posthumously. Sombra himself is treated more as this looming threat in the vein as Sauron in the Third Age of Middle Earth)

@Background Pony #D531  
And the most pathetic thus far.

Nightmare Moon was the most dangerous out of any of the villains so far.

@Background Pony #D91C  
Especially Chrissy :3
 
Sorry, had to say it XD

Awesome. Especially Nightmare Moon. <3