The most popular thing about Doctor Who has always been the Daleks. No matter which decade or generation, kids love them. They're an easily recognizable design, which ended up as toys that can be sold. When the show returned in 2005, it introduced a whole new generation of children to the Daleks and their popularity has rekindled and continued to blaze and soar. The toy sales are consistent. The Daleks are like Doctor Who's "Pokemon" if Pokemon were inspired by the Nazis. They're evil, but kids love monsters. They're safe spaces to explore evil for kids. And they have a kick-ass toy design.
Yes, Daleks were inspired by Nazis. Virtually every Sixties British genre show was inspired by Nazis and antifascist in message because the writers lived or even fought WWII. Their creator Terry Nation was certainly of that generation. When you look at them, the Dalek design was "tin-pot" as a mockery of tin-pot despots. The Daleks' monosyllabic voices and stiff accents were directly inspired by stereotypical Nazi soldiers' German accents. Their genocidal xenophobia was directly from the Nazis' antisemitism and desire to exterminate any race that wasn't Dalek. Their only mission was to wipe out all other races in the name of Dalek genetic purity. They force other races to become their temporary slaves until they're no longer useful, then they exterminate them. None of it was subtle. The Daleks were always designed to be the Ultimate Space Nazis. It's their stiff, shrill mean voices screaming the same things that really define them. Doctor Who has always been an antifascist show.
British genre TV in the 1960s and 1970s was often suffused with antifascist themes. The heroes were up against fascists and wannabe Nazis all the time. You couldn't get more convenient villains than Nazis. By the 1980s and 1990s, Nazis became kitsch, but now they've had a resurgence worldwide. The Daleks are suddenly a good analogy for teaching kids that fascism is bad, more than ever now.