For me, it's some kind of a turning point in Dean's story line. Up to this episode, he denied being afraid of dying. But then, confronted with "himself", he had somewhat an epiphany and wants Sam to find a way out of his deal. (yes I admit, I'm a Dean!Girl all the way!)

Further favorite episodes:

"No Rest Of The Wicked"
"Mystery Spot" (bittersweet funny)
"Ghostfacers"
"Red Sky at Morning" (if only for seeing the boys in tuxedos)

It's got to be either "Mystery Spot" or "No Rest Of The Wicked". But I picked "Mystery Spot", one of the best episode of the series (imo)
Hilarious moments: The scene in the diner when Sam is trying to prove to Dean that he knows what is going to happen, and says all these statements about himself in unison with Dean. Epic
Memorable Quotes: "Sammy, I get all tingly when you take control like that." xD
And, who didn’t enjoy all the creative ways they killed Dean? "Do these tacos taste funny to you?"
This entire episode is funny (ok, it's dark humor), but it's also an expression of Sam's love for his brother.
Rise and shine Sammy!

Also, I like "A Very Supernatural Christmas". I loved the scenes of Sam and Dean when they were kids ♥

It was the very first Supernaural episode I watched. It's special to me. ^_^

+ bad day at black rock, jus in bello, the mag 7

… As well as Fresh Blood, Mystery Spot, Jus in Bello, and No Rest for the Wicked.