So, who is your favorite character in the show, and what's your favorite episode? Why?
Holy crap if I really had to expound on this I would crash the forums with a manifesto of my feelings for Castiel, How I Used To Be A Dean Girl But Not Anymore, why I am disappointed with Sera Gamble's take on the show, my favourite fanfics and why (this would result in my computer short-circuiting on account of the massive outpouring of snot and tears) and of course, my favourite moments on the show.
I especially love the ones with Gabriel in it, especially Mystery Spot and Changing Channels. Oh gods, the Herpexia ad!! I also love all the meta episodes, especially the one where Dean and Sam attend their first SPN convention and meet all the fans and cosplayers. It was a fantastic way for the show's producers to give a shout out (and poke fun) at the fandom. The scene where the boys discover the frightening world of slash fic probably indulged every fic writer's (I admit to being one) personal fantasy - I mean come on, at some point we much have gotten off imagining our the characters reacting to the fandom and our stories about them.

The French Mistake was also comedic gold, there were so many awesome scenes - Dean and Sam's "acting", Misha crying and getting his throat slit, Sam finding out that he's married to Ruby Who's Not Evil etc.
And then there are the episodes that made me curl up and weep: A Very Supernatural Christmas, The End (cue the epic Castiel .gif to convey my feelings), On The Head of A Pin when Dean realises that he broke the first seal, Reading Is Fundamental when Cas was remembering who he was (AND THOSE MEMORIES STARTED WITH DEAN ;___;), The Man Who Would Be King, Swan Song and Death's Door. Sigh. The road to hell is really paved with good intentions, and I feel that it's one of the biggest themes of tragedy in SPN, where its characters are perpetually entrapped in a vicious cycle of self-sacrificing acts.
My favourite arc was that of the apocalypse, it felt that everything in the first few seasons was building up to the End of Days and fulfilling the prophecies. In that respect a part of me wished the show ended on a high at S5 with that deliciously ambiguous twist regarding Chuck. S6 and 7 felt a bit like they were stretching things out and flogging a dead horse, and I don't even want to talk about how disappointed I am with the Leviathans as the primary season villain.
BLEACH?! SERIOUSLY?!