Honestly, I started tuning out of GoT by the end of season 4, then 5 and 6 happened, and it became clear D&D couldn't write their way out of a wet paper bag without George R R Martin's source material to go off of. Not that Song of Ice and Fire is particularly profound or anything, but Jesus did the show fall off by comparison.
I think it was the Dorne and Winterfell arcs that were the last straws for me, though up to that point, the way they were handling almost every character was atrocious and should have been warning sign enough, not to mention all the fast travel, awful battle scenes, and flagrant inconsistencies in the political realm. At this point, the show is glorified fanfiction, complete with Mary Sues, lemons, and adolescent writing.