9. The Slitheen
First seen in Aliens in London during the revival series of Doctor Who, the Slitheen were a family of galactic criminals who sought to take over the Earth so that they could sell it to the highest bidder. Their wide-eyed, bloated appearance, and sharp claws could be interpreted as a design triumph, and the fact that they wore human skins as suits was a suitably nightmarish concept for a classic Doctor Who villain, had it not been for the fact that you could see the obvious zip across the forehead.
There were however some moments that showed potential. Particularly in the episode Boom Town, that focused on one of the survivors. The fantastic dialogue and great acting were solid pros, but the legacy of the Slitheen soon resorted to flabby paunches and childish farting that while may have kept the children entertained, were not the traits that we would associate with a classic Doctor Who villain who we can't wait to see again.
Following their appearance in Series 1, the Slitheen then went on to appear in the spin-off show The Sarah Jane Adventures, but with an even smaller budget. The Slitheen were not done any favours. They then went on to make cameo appearances in later Doctor Who seasons, being present in The Pandorica Opens and part of the fleet surrounding Trenzalore in The Time of the Doctor, but after many attempts at trying to promote them as monsters, many Whovians would agree that the Slitheen shouldn’t be revisited any time soon.