Creepy, but not terrifying (I'm looking at you, Bloody Mary and Provenance). Intense Sam and Dean stuff, too But mostly it's the case itself that's interesting. That was one of the best things about season one: how important and interesting each of the individual cases were.
For all that season two stole my heart when I first watched it (and I really do need to sit down and rewatch the whole thing sometime, but that will have to wait), it's hard for me to pick out an individual episode. But Hunted has Gordon Walker (representative of the worst of SPN's race issues, but still a fascinating and complex character who deserves more than to just be the guy Sam and Dean could be if they were worse). And while I wasn't as wild about Ava as many fans seemed to be, she's still a delight. It has Sam finding out about the secret Dean has been keeping from them. It has Dean being the dedicated big brother in over his head that he is (in this season, at least).
No Rest for the Wicked
Season three works for me far better as a bridge between S2 and S4 than as a season in its own right, and that's probably why NRftW is my favorite single episode. Sam spending S3 trying to save Dean didn't particularly interesting--Sam failing, and failing in part because he does what Dean wants him to, refrains from using his powers? That's interesting. Also, I really like Ruby. And Sam reassuring Dean that everything's going to be okay wins for me as the most heartbreaking moment on the show, knowing how things are going to end.
When the Levee Breaks
My favorite episode, period. Everything comes to a head, and Sam and Dean have the falling out that they will never really recover from. Sam's hallucinations drawing on his past, present and future to reveal his motivations for his present actions. Dean's desperate attempts to to stop Sam from making the choices that he thinks will destroy Sam. Their final argument and fight. It all felt earned to me. I could believe that the past four seasons had been building up to this point.
Point of No Return
I can't figure out how to explain why I love this episode without just describing the plot. How do you not love this episode? Sometimes, belief is more powerful--and more interesting--than love.
Let It Bleed
Mostly by process of elimination, I have to admit. However, I find everything involving Lisa and Ben in this episode to be very interesting. When I first watched it, I was irritated by how Lisa and Ben were written off the show. The lack of agency they had in the mindwipe. In retrospect, I'm much more bothered about the fact that Lisa and Ben were written off the show than how. So much more could have been done with Dean having a civilian connection, even a family, but nope. I guess the kind of thing would interfere with the two-brothers-against-the-world-driving-a
Hello, Cruel World and The Girl Next Door
Tie! I just can't not love Sam's insanity and his belief he was still in Hell and Dean trying to hold it together when he's already barely able to cope himself and the scene where Dean tries to convince him this is reality. But The Girl Next Door has Amy Pond, my favorite one-shot SPN character, and a fascinating moral dilemma. I really want to poke Dean's eye out with a needle due to how he chose to handle said moral dilemma. I can't imagine a worse way of handling it than the way he handled it. But that's not a criticism of the episode, although it might be a criticism of the follow-up.
This is also mostly process of elimination. I like season eight a lot, but it seems like every episode that has something I love also has something I hate. Even Citizen Fang pisses me off in how it handles Martin Creaser. The part where his mental instability is presented as the reason for his creepy behavior--as if Martin was shown to be creepy like that in his first appearance. As if we haven't had plenty of creepy hunters who weren't mentally ill like Martin. Also, the part where Martin the creep is the only character to ever offer criticism of Benny's mass-murdering past (even Sam is only focused on the possibility of Benny killing in the present). That's a cheap trick. But the episode has a lot of good, too. Sam's face when he sees Amelia with her husband. The tension between Sam and Dean, every conversation about Benny shaped by both their history and their present problems.
Possibly a funny choice, since Sam, my favorite character, isn't really in it until the end. But we have Gadreel going from killing his enemy to his friend, Dean falling apart (and the fact that it's his own damn fault just makes me feel worse for him), the simple horror of the torture that Dean and Cas have to put Sam through, culminating in Crowley's brief possession of Sam. And Gadreel. Did I mention Gadreel? He's awesome. And Sam and Dean's quiet confrontation at the end is lovely.
Season ten started petering out pretty quickly, but demon Dean was awesome. Pretty much every scene with demon Dean rocks, from his attitude towards Lester (Dean is at his best when he's calling someone out for being an asshole--and the more of an asshole he's being himself during the same time period, the better), to making it very clear to Crowley where Crowley stands with him, to mocking Sam and Cole.