9. The Highlanders - All episodes
The Highlanders is a very significant story for two reasons. First, it introduces Jamie McCrimmon. Played by Frazer Hines, Jamie went on to become the Second Doctor's longest-serving companion. It's easy to see why - while not the smartest of companions, Jamie is someone with a big heart who will do anything to protect his friends. Decades later, he still stands out as one of the best, thanks to consistently strong writing for the character and Hines's performance.
The other reason why The Highlanders is so important - it's the last "pure" historical story of the black-and-white era. In fact, with the strange exception of Fifth Doctor story Black Orchid - which hardly feels like a historical in the usual sense - The Highlanders is one of the last ever historical stories made with no science-fiction elements (other than the TARDIS crew, of course).
It's a great shame that the historicals were phased out. While monsters are generally seen as a big part of Doctor Who these days, the historicals were a great way of adding some more human-based drama to the series, at least when they were at their best.
The Highlanders might not be the best example of that, however. There are moments that come across as being too light-hearted and silly for its own good, especially given the historical context. But it would still be exciting to be able to watch one of the last historicals and the introduction of one of Doctor Who's best companions.