I am
so much behind everyone else but i finally got the DVD set and watched the whole thing in 2 sittings so it's quite difficult to pick a fave right now.
But it would be one of
Objects in Space - for all the reasons mentioned
Out of Gas - loved having some background on the characters and Kaylee was so great.
Ariel - generally just a good story. Especially liked that fact that Jayne betrayed Simon and River, but Simon is so naive he couldn't stop thanking him for getting them outta there!
Jaynestown was just hysterical. Loved the "Hero Of Canton"