For me it's probably "The End" where poor fallen Castiel is staying with Future!Dean even though the guy has clearly emotionally disconnected from it all. And then he is so happy to see "his" Dean once more. Also, when Castiel pulled Dean out from Zach's grasp there and at the roadside, for a second there I thought they'd hug. *draws hearts*
no subject
I love their relationship so much.