After I started thinking about it more, The Long Con might be higher on my list than this but whatever, I'm putting it here at #13. This episode was absolutely fantastic and truly fleshed out who and what Sawyer really is.
Not only does it get to the heart of the character by exploring his life as a con man but it ends with the twist of him playing Cassidy after an episode of having us thinking he wasn't. But this doesn't mean we saw Sawyer being the bad guy that he tries to be, we actually see the first inklings of what Sawyer really is. It kills him to do that to Cassiday and Josh Holloway's performance is top notch and one of his best in the entire show. When he sits in the car and we see that no one is in there with him, there doesn't need to be words, we know what Sawyer is thinking about feeling and its brilliant.
This doesn't make my list because of the WTF moment at the end, it makes it because it humanizes Sawyer, and grounds him in the real world with his one real connection; Cassidy.