If I had to choose one episode of Firefly I'd probably go with the pilot. It introduces all the characters very well and hooked me.
But, if I wanted to choose a non-pilot episode, I'd go with Jaynestown. Everyone needs to know the story of "The Hero of Canton".
Of course I would just tell anyone interested to watch the whole series in one shot, like I've done multiple times.
That’s how I first watched it (after it was cancelled, and oddly enough, after I saw Serenity). I think like CMT or some **** ran a marathon and I watched it all. I mean, it’s only like, 12 total episodes.