3. The Daleks
BBCCertainly the most influential and important character introduction in the entire history of the show, The Daleks' first appearance owes it success largely to unfortunate circumstances. Specifically, when it came time to film the cliffhanger for the end of The Dead Planet the Dalek props weren't ready. What was ready was a plunger. That was it. So in the grand television spirit of making do, they decided to use that one little bit of Dalek the best that they could and adjusted things so that it was a Dalek point of view shot with only the plunger arm visible ion screen. Which was about four billion times more interesting and compelling than any shot featuring an entire Dalek could possibly have been. It's the same principle that makes the shots of the alien in Alien so effective - the less you actually show and the more you hint at the better and scarier your monster is. CGI has sadly allowed a lot of directors to forget this principle, but it remains undeniably true. Your imagination has a much bigger budget than the production company does.