One I like from season 5 is when Jack asks Ben, "How can you read?" as they are waiting for their plane to crash and Ben says, "My mother taught me" and then gives Jack a little smile. Brilliant.
Another is this exchange between Charlie and Mr. Eko:
Mr. Eko: Climb that tree.
Mr. Eko: Climb that tree and perhaps we'll be able to get your bearings or see the plane.
Charlie: You climb it! What if I don't? You gonna beat me with your Jesus-stick?! I find it a little odd that your scripture-stick has dried blood on it!
Mr. Eko: Are you going to climb that tree or not?
[Eko gives Charlie a scary look and Charlie begins to climb]
Charlie: What kind of priest are you anyway?