I should just nod and point at Plei's list. However. My attention is required elsewhere, so I'll have to finish this later.
S1--
These first two were directed by David Nutter, well-known as a director of pilots. He is a master at laying out plot direction and giving broad but recognizable character notes in a single episode, and they had him for the first two.
Pilot-sets up the whole show, as Kripke envisioned it: The Sam Show, with sidekick wisecracking Han Solo older brother. We get the family history, that they were raised like warriors, that Sam opted out, that there's something wrong, Dad's missing, and a creepy mystery to solve. We get family dynamics and the mechanics of ghost hunting, and an emotional hook at the end. This show isn't going to pull punches and solve things happily by the time credits roll.
Wendigo-okay, the monster is a huge cheese factor, but it's set off by the cool points of CKR, so I figure those two balance out. Everything else is a plus. I love competent Sammy, his training taking over despite his reluctance toward the life. We get You gotta get a handle on that anger, man, it's going to eat you up. We get "This book." We get, "Killing things, saving people, the family business." We get smart Sam, figuring out what they're up against. We get frantic Sam when Dean gets got. We get a pretty elegant smoking gun, "What do you mean, I didn't bring provisions?" and a clever breadcrumb trick. We get Sam rescuing Dean. There is nothing about this episode I don't like.
Dead in the Water-Lots of backstory--Sam learns something he never knew about Dean, and we learn that Dean isn't just a wisecracking shady drifter who hits on anything. There's genuine concern in him for the defenseless, and that motivates him as much as pleasing his dad. It's also Kim Manners' debut as SPN director, and it is a turning point in the direction the whole series took. Up till this ep it was two guys in a muscle car chasing occult bad guys. Manners saw depth and complexity in what the actors could do with the characters, and nobody everlooked back.
Faith-Fierce determined Sammy, absent father, resigned-to-his-fate Dean. More detailed looks into the boys' psyches, what drives them, what their fears are, and more family dynamic.
Home-Ultimate backstory, and more backbone on which to build mytharc.
Something Wicked-backstory. Sam learns more about Dean
Shadow-Dad, of course, and Meg.
Dead Man's Blood, Salvation, Devil's Trap-a three-hour episode. So much family and character goodness I can't eliminate any of them.
(I give nods to Phantom Traveler for Dean's glee over the EMF meter he built and Sam's disdain for it, for the depth of Dean's flying phobia and the ease with which Sam powers over Dean's fear and helps him (makes him) deal with it. And for the backstory that John and Dean had helped someone who'd remembered and called on them again in need. And to Bugs for the Dad did check up on you at Stanford, moron. Did you think we'd let you go off and forget you?)
S2-
IMTOD-Arguably one of the best eps of the series.
Children Shouldn't Play with Dead Things-Sam trying to pull Dean back from self-destruction. I'm not fond of the zombie storyline, but there are so many good moments in this ep.
Simon Said-I love Andy. It also escalates Sam's jeopardy with the YED, and showcases Dean's fear for Sam and determination to protect and save him.
Crossroad Blues-sets up the deals later on. Plus bonus points for use of the Robert Parker legend.
Croatoan-what Amy said, plus the ending scene.
Hunted-the bookending opening scene. Introducing Ava. Funkytown. Gordon's fervid belief in Sam's eviltude.
Nightshifter-everything
Houses of the Holy-more backstory. Sam's desperate belief. Dean's desolate denial.
Born Under a Bad Sign-Oh, Sammy. Oh, Jo. Oh, Dean.
Heart-With the Save a Monster, Save Myself. Oh, Sam.
what is and What Should Never Be-Oh, Dean (Also, I watch The Forgotten *just* for Michele Borth. I love Carmen and Dean together).
AHBL--a (continued...)