Season 6, Episode 9: "Battle of the Bastards"
The title tells the tale on this one — Jon Snow vs. Ramsay Bolton in a gritty battle for the fate of Winterfell. Along the way, hundreds die, including Shaggydog, Rickon Stark, and Wun-Wun the giant. Yet what's most notable here is that Sansa Stark ultimately saves the outmatched Northern forces and helps win the battle by calling upon Littlefinger to bring in the Knights of the Vale. She then enacts her revenge against Ramsay by feeding him to his own hounds.