my favorite was s8 last episode in the church when Sam told Dean that he confessed that he didn't want to let Dean down and Dean saying that there was no me without you I cried so much because you saw the love Dean has for his brother
it's hard to nab it down to one, but if I had to recommend one, it would be "The End". Mostly due to the fact that Jensen did an amazing job of playing the two versions of Dean, and alone for that I could watch it over and over again. But also for Jared's short portrait of Lucifer (or Sammifer, as the fandom called it), who gave the fallen angel a noble side, which we never saw on any other demon. And last but not least for Hippie Cas, which gave Misha the opportunity to show a bit more of his acting skills as well
