3. The Tenth Planet - 1/4 Episodes Missing
With only one episode missing, in some cases this wouldn't seem too serious and other stories would be considered a much higher priority in terms of recovery. However, the one missing episode is very significant, as this one episode contains the first ever regeneration. The Tenth Planet introduces the Cybermen as they attempt to invade Earth and convert everybody to be like them, on account of their home planet is dying. This would become the plot for many Cyberman stories to follow. However this story did it first and many would argue, best. The regeneration itself does exist as some very poor quality footage. However, the whole episode would complete a truly chilling story and allow us to see a true moment of televisual history in its full context. It would also allow us to see a little more of Polly and Ben, who don't actually have a complete story in the archives currently (although admittedly The War Machines is only missing a few seconds).