I'd have to go with Winds of Winter. The opening scene with the Light of the Seven theme playing was beautiful and so damn eerie at the same time. That was some damn compelling television. I also loved the Tower of Joy scene and the reveal of Jon Snow's lineage.
A close second would be The Rains of Castamere. I'm not a book reader yet (will be changing that soon), so I was completely taken off guard by what I was witnessing. Not to short any of the other episodes, I think these five are mostly unanimous as the highest quality GoT episodes HBO has made. (I understand there are exceptions; The Door, Blackwater.) Each person is different, I've talked to many who wouldn't list any one of these five episodes as their favorite. And if I'm honest, The Spoils of War is maybe an above average episode without the last 15 minutes. The Loot Train Battle arguably got everyone off (as it should've) and it skyrocketed its IMDB score. Certain great scenes, and there are too many to count throughout all 7 seasons so far, can really change a viewer's perception of the entire episode.
Maybe this has been discussed before in the other thread, I can't remember. Just wanted to see what everyone's thoughts were.