38. The Second Doctor Meets the Ice Warriors
From: "The Ice Warriors"
Premiere: November 11, 1967
It's surprising that the Ice Warriors became an iconic Doctor Who threat, given the scarcity of their actual screen time. Introduced in this gripping six-part serial – two episodes of which were lost to frugal BBC disposal habits that saw numerous early segments destroyed, with hunters scanning the globe for remaining copies to this day – the lumbering, tortoise-shelled, hissing soldiers from Mars proved memorable despite turning up just three more times before their modern resurrection in 2013's claustrophobic "Cold War."