The “jump the shark” moment for me
The nonsensical zombie quest which served ZERO purpose (Cersei never even allied with them after being shown the zombie) / dragon being conveniently killed so the Night King has some way of breaching the Wall (what was his plan if nobody brought a dragon to him?) / Benjen appearing out of nowhere to rescue Jon (big deus ex machina) / the huge chain the zombies just happened to be carrying / Tormund and Beric surviving a nuclear strike.
It also played complete havoc with the geography of the show (ie the speed with which Gendry, the ravens and the dragons travelled).
I think a lot of people gave this episode a pass because, visually, it looked ridiculously cool. Which I think was I intentional to distract everyone from the genuinely cheap writing.