8. The Slitheen - Aliens Of London/World War III
The Slitheen--or "Sliveen," as Rose Tyler might say--are the villains of the Season 1 two-parter "Aliens of London"/"World War III," in which aliens from Raxacoricofallapatorius capable of zipping themselves into human skins basically take over the British government from the inside. While the implications of their mass-murdering spree are lessened by the ongoing flatulence gag, the Slitheen were a formidable enemy and the threat that they posed to Nine, Rose, and M.P. from Flydale North Harriet Jones was real. However, if the Abzorbaloff is any indication, a creature that tries to rely too much on bodily humor loses its appeal very quickly. The Slitheen have been absent from the series since the end of Season 1, and they should stay that way. Still, if you say that you haven't tried to say "Raxacoricofallapatorius" out loud, then you're either lying or going to try to do it now. Note: it's a very satisfying accomplishment and you should definitely do it.