I have been stuck on this prompt for weeks. The pilots of so many of the my favorite shows rock, but at the same time, most of them aren't nearly as good as later episodes, so they're not particularly dear to my heart.
I guess I'm gonna have to go with "Pilot," the Lost pilot. It's mesmerizing and memorable, a plane wreck you can't look away from. And it includes that show-defining quote from Charlie: "Guys, where are we?"
In fact, as soon as Nanowrimo ends, I'm planning to rewatch the pilot while it's still available for free here on hulu.com.