Only counting the original 14 songs, I'd say Superpower or Pretty Hurts. No Angel used to be the worst for me, but it's grown on me as time has gone by.
Pretty Hurts is definitely my least favorite track on here. It's the only one on self-titled that I wouldn't consider a "good" song. It's just very cliche and lackluster.
obviously if we are counting bonus tracks Ring Off is the worst. but SOTS Remix wins as far as being the best, its one of her best songs to date in general