The worst episode of Game Of Thrones and why?

This is absolutely no contest. It’s clearly S5E6: Unbowed, Unbent, Unbroken.

Let’s see:

• The Arya scenes were the only decent part of the episode, although at times it was hard to see anything (the scenes were very dark). The Hall of Faces looked good but reportedly, they had created unique designs for each and every face, which made me wonder if this was really the best allocation of budget and resources.
• Further whitewashing of Saint Tyrion and another cock joke that does not belong (in this context) in a show that is supposed to take itself seriously.

Let’s go to Dorne, shall we?

• Jaime admits to Bronn that he doesn’t have a plan.
• The Sand Snakes and Jaime & Bronn just happen to (try to) kidnap Myrcella at the same time.
• All of Dorne is reduced to one tiny garden, a desert and Doran’s house
• Dorne consists entirely of stereotypes, Trystane included
• We got the most horrible fight choreography in the history of television:

• Did I mention that the fight was really, really bad? Perhaps the worst part was that there was zero tension. It was pure comedy and it never felt like any of the characters would even experience pain, let alone get killed. It effectively ruined the Dorne plot, irredeemably.
• Areo Hotah never got to use his axe (like, ever)
• Apart from the Sand Snakes being captured, nothing really happened here. This could’ve easily been reduced to a very short scene with Jaime and Bronn arriving (and with the Sand Snakes already present).

Back to King’s Landing:

• Loras being gay is his only defining characteristic in the show and now we are supposed to believe that the Faith is extremely upset over this. It’s not like he had sex on an altar or something.
• Apparently the Faith Militant overthrew King’s Landing but we don’t really know how and no one talks about it. I mean, there should still be a 30k+ Lannister/Reach army, right? So why does anyone care about a bunch of priests with wooden clubs?
• They literally use a birthmark on someone’s buttocks as the main evidence. Something which could easily be common knowledge (for example, a squire or fellow soldier would surely notice it).
• Margaery is arrested for giving one false testimony. Was that really the best the show writers could think of? Why not have Cersei set her up in some way?
• Littlefinger can send a raven to Bolton to discuss Sansa’s wedding but now he needs to teleport to King’s Landing and tell Cersei in person?

And now the worst part, Winterfell:

• The entire scene with Sansa’s wedding and rape adds absolutely nothing to the plot. Its only purpose is shock value.
• The show writers literally scrapped every Winterfell arc from the books (the ghost of Winterfell, Mance, Jeyne, the North Remembers) and the only part that they felt should be adopted to television, was that someone should be raped by Ramsay.
• We already knew Ramsay was a sadist so it didn’t add anything to his character arc.
• Similar for Sansa, she had already suffered enough in King’s Landing. She didn’t need any further abuse. She was actually on her way to control her own fate, until Littlefinger dropped her here of course.
• Even for Littlefinger’s arc, this scene was useless since Sansa already knew that she couldn’t trust him. Another ‘Littlefinger betrays Sansa’ moment wasn’t necessary. This is especially obvious after the Season 7 finale. It’s also inexplainable how Littlefinger, a man who values knowledge over everything else, didn’t know about Ramsay.
• Essentially, this episode puts Sansa back to Season 1–4 and halts her character growth.

• The show writers almost seemed to suggest that this scene was empowering Sansa. They gave her a dark dress (previous episode) and had her manipulate Littlefinger. Just like Jaime’s rape of Cersei next to Joffrey’s body (which they later claimed was supposed to be consensual), this shows that sometimes there is a huge disconnect between what the show writers intend to do and how it is shown on television.
• The entire purpose of this scene in the books, is that it happens to a ‘nobody’, which is why no one cares except for Theon. These themes/characterizations are completely reversed on the show.
• The whole rape scene is all about Theon. So, even when she is being raped, Sansa is deprived of being the main actor in the scene. Now, let me be clear: I am not saying there should be more gratuitous violence/nudity here. But they could’ve shown Sansa’s response afterwards, but instead it’s all about how Theon responds to it.
• Everything that happened in the last episode, with Sansa saying ‘you can’t frighten me’ to Myranda and ‘this is my home’, turned out to be meaningless.
• I actually expected Ramsay, as he is portrayed in the show, to be much more gentle here. He was always playing the kindly lord around her before, so why did that suddenly change? ‘Because he is a sadist’, again, adds nothing to his character arc and doesn’t make him a more compelling villain.
• In short: the entire scene was only added because the show writers wanted to have Sansa raped and it was seemingly pushed as a necessary moment for Theon to redeem himself and for Sansa to ‘evolve’. If that isn’t downright offensive and sexist, I don’t know what is.

Some close contenders:

S7E6: Beyond the Wall, as already mentioned in other answers here

S6E8: No One, with the culmination of Arya’s arc in Braavos and the Blackfish dying off-screen.

S5E9: The Dance of Dragons, with Shireen’s burning while Stannis watches, Dorne filler and Daenerys as a damsel-in-distress in the Fighting Pit.

S7E7: The Dragon and the Wolf, just overall a very predictable finale lacking any suspension or surprising twists whatsoever

S4E3: Breaker of Chains, where Jaime rapes Cersei in front of their dead son.