So, what is the best episode of your series of choice to get someone hooked? It should be good, obviously, and fairly stand-alone so that newbies won’t be confused.
Babylon 5: Passing through Gethsemane - a great sample of the cerebral style of B5
ST: DS9: Homefront - One of the best uses of the Dominion as a cause of paranoia.
Firefly: Serenity (pilot)
Good Eats: Tender is the Loin (probably the first, not the second one). I think that Alton is cute, but not over-the-top in his demonstrations, and gets some good cookin’ done on something that people love, but don’t cook too often.
The Wire* – Season 1, Episode 4, “Old Cases”. Listening to Bunk and McNulty “communicate” as they revisit an old crime scene will crack your shit up, yo.
*I just started watching. What a great show.
ST:TOS: City on the Edge of Forever
Simply the best episode there is. And one doesn’t need to much about the Trekiverse to enjoy it.
Arrested Development: Fakin’ It
William Hung and the Hung Jury. 'Nuff said.
The Office (U.S.): Casino Night.
Every major and minor character gets at least one good moment.
Lost: Walkabout. The first Locke flashback, that was where we realize the island was something special, and Locke’s personality, showcased well in that episode, is more important to the show’s dynamic than any of the other characters’.
For Sopranos, I would suggest watching the whole thing straight through because it’s easier to understand. But for a stand alone episode to encourage someone to pick up season one, I suggest season three’s Pine Barrens. Fairly easy to understand, as it takes two of the characters into a strange environment (New Jersey’s Pine Barrens) so there aren’t as many characters to keep track of, but plenty of great one liners/examples of good character development.
Buffy the Vampire Slayer - Hush Not at all the typical episode, with almost zero dialogue (which is what the show was known for). But a perfect capture of everything that was Buffy.
Angel - Either City of… or Reprise. The pilot set up everything so nicely, but Reprise was when Angel realized that the fight isn’t about winning, it’s about doing what’s right because somebody has to, win or lose.
Grey’s Anatomy: Watch “It’s The End of the World (As We Know It)” and “And I Feel Fine”, the two-parter from season 2. It shows the entire cast, from their best to their worst, and it brings all the threads of the season together.
Rome: Watch the pilot. That’s what got me hooked. 
Supernatural: Watch the season 2 finale, “All Hell Breaks Loose (parts 1 and 2)”. It’s got the show mythology, it’s got emotional trauma and upheaval, it’s got action and creepiness, and it’s got hot guys. Lots of them. 
As for Lost, I’d actually argue for “Live Together, Die Alone”–there’s everything: the A-Team, the Others, the first appearance of Ben (as opposed to Henry Gale), Desmond and the hatch…
House: Three Stories.
The Simpsons - The City of New York vs. Homer Simpson
Futurama: A Fishful of Dollars. It’s not touching like Jurassic Bark or Luck of the Fryrish, but it’s a beautiful blend of the entire cast in great comedic moments - Fry, Mom and Zoidberg totally shine.
There’s practically one for each season of The Sopranos, but if I had to pick just one episode, it would be “College” from season 1. Meadow and Tony and the rat for the main course with a nice side of Carm and Father Phil.
Moonlighting: Atomic Shakespeare.
Pure comedy gold.
Aqua Teen Hunger Force: Dumber Dolls
Magnum PI: The one where Thomas shoots the bad guy at the end, just because.
Twin Peaks: The first episode. Forget the rest.
King of the Hill: Hank the Pimp. “Alabaster!” You’ll wet yourself laughing.
South Park: Cripple Fight.
South Park: Starvin’ Marvin
*Venture Brothers*: Escape to the House of Mummies: Part II (FTR, there is no part I or III.) It epitomizes everything VB represents: failure (both on the part of Dr.Venture failing to save his family and to fix the shrink ray, and in Dr. O in failing to shrink,) Dr. O’s grandoise style, the complete craziness that the family gets into (with Brock, et al.) Brock smashing shit up, the boys almost dying, etc…
Gargoyles: Future Tense
Smallville: Zod (Hey, they brought back the “kneel before Zod,” you gotta love that.)
Damn, I came into this thread to say that! Excellent episode (and a Stacy one to boot; I didn’t care for Vogler, didn’t care for Tritter, but man I love Stacy.)
Seinfeld: The Marine Biologist (with the Soup Nazi a close second.)
West Wing: In Excelsis Deo
The series was full of remarkable episodes, but this one is going to move you. (Interestingly the White House is not the centre of the main storyline.)
CSI (Las Vegas): Chaos Theory
Methodical investigation at it’s best.