It was a hard choice for me, because I love this one but I also love Swan Song so much. In the end (ha ha) The End won my vote because when I watch it, I watch all of it. In Swan Song, I tend to skip the Lucifer/Sam mirror thing. But there isn't a minute of The End that I don't want to watch: Dean breaking Sam's heart on the phone, his awkward conversation with Cas, running from Croats, Dean meeting Dean, Future!Cas meeting Dean, Samifer /o\, the adorable Dean/Cas conversation when Cas brings him back, and the reunion with Sam. It's all so good. Normally I wouldn't pick an episode with so little Sam as a favorite, but in this one, his absence is necessary and meaningful.
And I know I said this earlier, but the three leads do such an amazing job playing not quite their normal characters. Look at Jared's face. That is not Sam.
And there's the thigh holster too, which is also kind of important.
Thanks for voting!