I do love Out of Gas. The comedy and sweetness of the flashbacks contrasts wonderfully with the mortal tension in the present. It could be a great pilot episode, but its even stronger for the fact that you already know these characters.
Objects in Space comes a close second in my reckoning, mainly for that soulful and personal commentary track by Whedon.
One (of the few) that hasn't been mentioned so far* is Janestown, which has a few of my favourite moments. Mal has one of those lines that stays with you, "It's my estimation that...every man ever got a statue made of him, was one kind of sumbitch or another."
*EDIT: Actually, it has - in the first post no less!
Last edited by mikhail; 14-07-2012 at 12:29.