I've no idea why y'all are saying "Expose." I hated that episode, and thought outside of "Fire + Water" and "Stranger in a Strange Land" it was the worst episode of the series.

My favorite episodes:

"Walkabout"
"The Man Behind the Curtain"
"Through the Looking Glass"
"The Constant"
"The Shape of Things to Come"
"The Life and Death of Jeremy Bentham"
"Ab Aeterno"
"The End"