For the first time ever I'm going to rank the series. Everybody else on the internet is doing it so I don't have an excuse to stay behind.
From best to least-best (there aren't any bad ones).
1. Deep Breath (1)
It's a toss-up between the first and last episode, but because I'm still emotionally reeling from the last one I think that gives it an unfair advantage.
2. Death In Heaven (12)
Like I said it's terrifice and that opinion is never going to change. Perhaps I should adopt James' opinion-expiration-date theory but I don't have to worry about that right now.
3. Last Christmas (special, and timey-wimey closer to series 8 than to series 9. And it's also the way Prime does it).
4. Flatline (9)
I love crazy stories about dimensions and it's also the kind of funny-but-oh-so-creepy stuff I would except to see in a 1970s episode of Doctor Who. Whether that's going to happen or not remains to be seen, of course.

Lately I have been wondering if I'm going to recognise the scenes from a 1970s episode I've watched together with my brother. I didn't really understand what I was watching but I felt there was
something about it, and that feeling has always stayed with me.
It's such a vivid memory, I can see myself sitting on the couch watching the television, and it was something about a character (possibly the Doctor) walking around and examinating some kind of object, maybe a big glass ball with something in it.
But maybe that's just the distorted remains of that memory.
5. Dark Water (11)
Yet another episode that highlights what a terrific companion Clara is. That blackmail scene with the keys is just so very tense and audacious and both ugly and beautiful.
Sometimes series 8 feels like what TNT Dallas was to DALLAS.
6. Into The Dalek (2)
Because every great Dalek story is winner to me. It's impressive to see how they keep finding new angles to re-introduce the iconic villains.
7, Kill The Moon (7)
Regardless of the preposterous conclusion I still like the horror spiders even if they weren't spiders, and that unexpectedly grim and emotional ending (à la John Ross and Sue Ellen's famous kitchen scene) gives it the edge over the next rated episodes.
8. Mummy On The Orient Express (8)
They must have felt very smart about themselves when they came up with this title - and rightfully so - and the entertainment is as non-stop as the space train itself.
The disinterested attitude towards the death of various passengers provides the black comedy.
9. Robot Of Sherwood (3)
I always like it when they visit historical and mythical characters, and it has a good villain with an equally dastardly plan.
I particularly like the moment when the Doctor, determined to prove that Robin is a fraud, confronts Robin with the future results of that very fairy tale.

It was a nasty thing to do and I felt sorry for the handsome guest star hero.
10. In The Forest Of The Night (10)
Yes, it's a bit soft and wholesome, but I don't mind that the series does something kids-friendly every now and then. And I must say I really enjoyed the child actors in this episode.
At some point Clara (or Danny) says to one of the school kids "you're just imagining things" (as opposed to...a spontaneous global mega forest? So funny that they always try be logical in the most absurd situations).
But anyway, the child responds with "that's not possible because I don't have any imagination".
The miraculous return of missing sister Abigail was a bit too schmaltzy for my taste, it would have been better to let that part of the story linger on, no matter how dark it is. Or end it with a last second twist, only for the viewers, that Abigail is now an evil alien.
11. Time Heist (5)
I find it a very beautiful and intriguing story, but the combination of complexity and standard episode running time leaves something to be desired. Two characters who died had survived, and they mentioned how (in just three or four words) and I just didn't understand it.
And there's no time to try to digest it because the next thing that happens requires all my attention.
Had it been a two-parter (and perferably less plot-holey) then maybe it could have been as great as David Tennant's magnificent "Library" story.
12. The Caretaker (6)
Comical fluff with a not too exciting threat. But it was nice to create some time for Clara and Danny, and that also had to be part of the series.
13. Listen (4)
I found it almost instantly forgettable, but I think there was something in it about the Doctor as a child, and I don't really need to know everything about all the origins. He's a time traveller in a blue police box, that's it. I feel the origins stuff works better for the Master.