Personally I think The Winds of Winter is the best episode, Battle of the Blackwater coming next.
The Winds of Winter is incredibly satisfying to watch after witnessing everything that's lead up to it, it was the episode that kept on giving, especially the opening sequence.
The actual battle in The Battle of the Bastards eclipsed the Blackwater battle by far, but the plot focused on 'Jon vs Ramsay', I just thought it was too contained/superficial and the Blackwater episode had a wider, more interesting scope with more depth/far better dialogue. I've seen S06E09 twice, and on the second viewing it had already lost a lot of its spark.
S02E09 and S06E09 both have my favorite cinematic moments of the show, the wildfire explosion/shield wall.