HA!!! How does anyone choose out so many good episodes?? I'm going to go with 1 Favourite Episode for EACH SEASON - it's the only way I can address this without my brain imploding from the decision-making overload.
Working Backwards:
Season 8: SACRIFICE - didn't even hesitate, without a doubt one of my all time favourite Sam and Dean scenes EVER.
Season 7: Slash Fiction
Season 6: I CANNOT DECIDE!! The Man Who Knew Too Much and The French Mistake
Season 5: The End and Swan Song
Season 4: When The Levee Breaks and The Monster At The End Of This Book
Season 3: A Very Supernatural Christmas
Season 2: All Hell Breaks Loose 1&2
Season 1: Pilot