I'm not mad they killed Stannis, I wasn't expecting him to win, but... I hate that the battle they'd been building up all season happened off-screen, and that Stannis was just dead a few minutes in. It felt rushed and anti-climactic as hell.
As for the rest, it was great. Theon finally getting a grip and escaping with Sansa was good, Arya killing Meryn was satisfying as hell, Myrcella dying was sad, Cersei's walk of shame was also satisfying, Robert Strong (Cleganebowl! Get hyped!), Daenerys getting captured by Khal Jhaqo (I assume it's the same as the books on this, but who knows) was interesting and of course... Jon's death. I knew it was coming since it happened in the books... but still sad.
What made it for me was Stannis' last words. "Do your duty". It was exactly how I'd pictured his death. No excuses, no justification, no begging.
Just do it. Stannis looked... done. Tired. Broken. Death was probably a relief.
All in all I'd rank it as 7/10, decent, but the Battle of Winterfell anti-climax really soured it for me.