The Waters of Mars: one of the most intelligent, original, well-written, genius episodes of TV ever created with an all time brilliant ending, some of the best character work of the revival and a strong contender Doctor Who’s best monster of the week.
The End of Time: Part One: a mistake.
At the risk of crearing a brand for myself (and because they haven’t been mentioned yet), the EDAs do this a lot. The rise in quality from The Eight Doctors to Vampire Science is massive and immediate. Pretty quickly after that you have Genocide, down to War of the Daleks, then back up to Alien Bodies. I’ve heard some people say that the first several books basically alternate between the best books of the series and the worst. I don’t quite agree, but I completely understand
There’s another huge whiplash moment from Unnatural History, down to Autumn Mist, then back up again with Interference. Honestly, the whole run from Demontage to The Taking of Planet 5 is brilliant, apart from Autumn Mist right in the middle. I’m not sure if that makes it better or worse
For single steps, the fall from The Turing Test to Endgame is pretty big, as is the one from The Year of Intelligent Tigers to The Slow Empire
An Unearthly Child, then the fall in quality of every single episode after that 
Good to bad: The God Complex followed by Closing Time.
Bad to good: 42 followed by Human Nature/The Family of Blood.
I actually don’t think the bad stories here are awful or anything but they are subpar IMHO.
Tad controversial, but imo Heaven Sent into Hell Bent is a prime example of a massive dip from good to bad
I like Hell Bent more than Heaven Sent. But I am not a massive fan of either of them 
I agree. And it amuses me that two of the best regarded stories feature dodgy monster moments - Genesis has the clams, Androzani has the bat creature. But neither of those are bad enough to weigh the rest of the story down, I think.
Interesting thread, and fun to see people’s opinions. I don’t think there’s a larger chasm than Caves → Dilemma, sadly.
The irony is, on its own in the middle of a season, I don’t think Dilemma would be regarded as badly as it is. Part of its damage is because of its placement (a week after Caves, debuting the new Doctor, and that serial being the lingering aftertaste for 9 months until Season 22). If it’d been wedged somewhere in the middle of Season 22, like Timelash, it’d still be pretty naff but it wouldn’t have anywhere near the terrible reputation it has, and I think people would be fairer on it.
Earthshock into Time-Flight isn’t a great transition either, and on reflection neither is Enlightenment into The King’s Demons. 5’s seasons don’t have great finales, huh.
Doom Coalition 3 is one of those boxsets where every story is better than the last seemingly
I love both Time-Flight and The King’s Demons
I love The King’s Demon.
I love Time Flight 
I’ve sometimes used the term “clam” to describe a relatively small issue that has been blown out of proportion, named after the clam in Genesis.
It was pretty much the opposite for me lol. Absent Friends was amazing and the rest…wasn’t
Every time I see the words Absent Friends I think about that audio that we’ll never get to hear 
I don’t think I know. Which audio?
Read ths for the sad story
I’ve not dived into torchwood yet, so it doesn’t mean a lot to me, but I’ll be sad for you
We could have had Ten in Torchwood
(I haven’t even listened to the Torchwood audios yet but I still get sad about it)