8. The Exciting Adventures Of Space Pig!
I personally am a huge fan of the Adherents of the Repeated Meme, should anyone ask, but if you were to ask the bulk of fandom who the best monster of season one (series one for you Euro-types) was, there's a pretty good chance that they're going to say 'Space Pig' (I know. Not the Slitheen. Shocking.) The year was 2005, and the story was "Aliens of London". (Someone please begin playing Warren Zevon at this time.) As part of the Slitheen's sinister plan*, they had to convince the world that we were under threat of alien attack. So they fake up an alien, chuck it at Big Ben and bingo - the World is on red alert. In order to fake up the 'Alien', they take a regular pig, rewire his brain a bit to be all alien-y, teach him to walk on his hind legs and put him in a space suit. The result was undeniable
adorable. I'm not sure if it was a deliberate homage to the Muppets' old Pigs in Space bit, but when he starts squealing while he briskly jogs down the hallway in his little outfit, I defy you to not think 'awwwwwww' to yourself. Which makes it a particularly effective sequence when Space Pig is brutally gunned down by the military a moment later. One heartbreaking speech from The Doctor about how scared he must have been and you have a truly touching, charming and heartbreaking scene about something fundamental in humanity. Centered around a pig in a spacesuit. I'd like to see Law and Order pull that one off.
*I'd like to take this moment to point out that for all the flack the Slitheen get, their sinister plan actually holds together pretty well and makes significantly more sense than anything the Master or the Cybermen have ever attempted. Say what you will about the childish farting, they'd at least bothered to put some time in on the forward planning.