Nick Lachey - What's Left Of Me > SoulO and I think he's third one might be even better.
Ashlee Simpson - Bittersweet World is her best album IMO, even though her first 2 are amazing as well.
Edit -
Paramore, how could I forget?

.
The not so sure section -
Kelly Clarkson - I I don't know if MD> Breakaway but in some areas it's def' an improvment.
Pink - All of her albums are amazing, I don't know if you can say she's getting better but she's def' not getting any worse.
The Veronicas - This is more like The Secret Life of...=Hook Me Up they've gone into a different direction but I love both, so might not improving but staying great is something right?