The Baddest of the Bad

Get out your party hats, or should I say protective helmets, because this week we will be looking at those characters who have played havoc on the various crews of Star Trek.
No hero comes without a foe, and the various incarnations of Star Trek have had their fair share of foes. But this week we leave it up to you to decide who was the baddest of the bad. Who made the largest impact on both you as a watcher and to those who had faced either him or her? Who made you think to yourself, while watching, “wow, that’s some baddy”?
As always, learning is fun. Especially when it comes to what makes for the most evil of the evils encountered in the galaxy. So head on down to the forums, and let us know who made you shake in your shoes when you first saw the evil (fill in the blank) and of course, why?