S3 E8 - A Very Supernatural Christmas: Young Dean and Sam exchanging presents
It's a total tearjerker and my heart aches for the young boys, but GODDAMNIT I WANNA STRANGLE JOHN WINCHESTER. This scene makes me hate that worst-father-ever with passion.
BTW, after remembering this episode, I'm totally ignoring my previous post and changing my "happy scene" into the one that comes straight after this flashback section. JFYI.