The Slitheen (or Raxacoricofallapatroians... nah, fuck it, just Slitheen) are the Doctor's greatest foe, his arch-nemesis. They fart, and they're the sexiest aliens ever. I'd definitely sex them. This is also the very first page of this mindblowingly autistic wiki.
Reminder to check out The Slitheen Wars.
Also on several occasions massacred by some children (At least they are not out of place there).