From her first album, listen to Knock 'em out, LDN, Smile, Not Big, Everything's wonderful, these are my faves. Her "Cheryl Tweedy" song is good as well, but its a Bside to her "Smile" single. Haha.
Her second album is awesome as well. Check out "The Fear", "Not Fair", "lovelovelovelove You" (imo the best song ever), "who'd have known", "22", "Never gonna Happen". But all the songs are equally amazing in this album actually.