Expose is up there. But I think that's a special episode, really :}~
The End because it was smurfing awesome.
S.O.S. because I love Rose and Bernard, especially the scene where Rose tells Bernard she's sick.
The Shape of Things to Come because it's smurfing awesome, and Alex's death was intense.
The Variable because Daniel Faraday was the best character D:
Lighthouse because it was awesome, and Jack's freakout was also awesome, and Jack is awesome.
A Tale of Two Cities because the flashbacks for Jack were smurfed.
Not in Portland and One of Us because Juliet. End of story. Juliet.
My favorite episode is The Other Woman, fo sho, though. Because Juliet. And Ben! And the creepy date. And the impaled Goodwin corpse. All crazy amounts of smurfery.
What a good show.