Series 1 to Series 2. Sorry, but series 1 has so many better stories like the finale and the Empty Child two parter (which is certainly better than The Girl in the Fireplace). It didn't have as many clunkers either. Even the worst episode from series 1, The Long Game, is considerably better than the likes of Fear Her and The Idiot's Latern.
If we are talking about the drop in quality for New Who as a show in its own right, then surely it must be between series 7 and 8.
Like any long running series it had its ups and downs before that sure, but honestly I don't think any show has collapsed that badly in such a short space of time.
It's not just even in terms of quality, but popularity and its fandom. In 2013, DW had never been more healthy. Let's see, 50th celebrations, which despite the controversy now was a success at the time. An Adventure in Space and Time, The Fivish Doctors Reboot, two lost Troughton holy grails, and on top of that there was so much good feeling for all of DW, not just the revival, but the old one too.
With Capaldi being cast meanwhile it seemed like we would be getting a proper old school Doctor, going on adventures. The last of the time lords angle had been wrapped up, Matt had managed to bridge the gap between the young Tennant, stud Doctor and the older, more alien, professorial Doctor for the new generation.
If Moffat had just not pandered so completely and utterly to one section of the audience, then there is no telling where it could have gone. Old and Classic Who fans were united in their love for it, a big name actor was in the part, and hey even if he did want to pander to the feminists, there were other ways of doing it. Bring back Romana now that Gallifrey is back, not sexualise the companion like he had done before with Rory looking up her skirt ffs.
However in ONE year Moffat flushed all of that down the toilet. With Missy, Cyber Brig, the Doctor being the President of Earth, he drove away almost all classic era fans. With the miserable, horrible, nasty aspects of stories like Death in Heaven he drove away the family audience, yet at the same time the show couldn't get a reputation as a dark and gritty series either, because it was so campy and stupid with things like Santa, moon eggs, magic trees bringing people back to life etc. It also completely misused Capaldi, having him be jealous of Clara's boyfriend and making her and Capaldi's relationship with each other nasty.
The show went from being literally the most popular in the world in 2013 after series 7, to being one that in 2015 after series 8, could barely pull in more than 3 million viewers, even when it had the Daleks and Davros and was on after Strictly Come Dancing, a show that got 11 million and was advertised on the side of buses and in cinemas! 8 million people were still actively switching off as a result of Death in Heaven.
It never recovered from that drop. Apart from a brief spurt for Jodie's first season thanks to morbid curiosity and youtubers needing something to hate watch.
Meanwhile the fandom whilst always having its problems, became a political battle ground after Pissy Missy's casting, which again anyone with half a brain could have told Moffat would have happened if he kept up with the gender bending bullshit.
Series 8 is what killed New Who stone dead. It's just been kept in a Zombified state ever since because the people who run it are in with the bricks and it has the correct agenda.
