Underrated: Sixth of the Dusk, The Final Empire
Overrated: The Emperor's Soul
SotD is a unique setting—a drastic departure from traditional fantasy locales—and presents a character type that's new for Sanderson. TFE, I think, is his second-best paced book (after BoM) and has the most satisfying plot of any of his books.
As for TES, I never understood the infatuation with it. It was a decent enough story, but I couldn't empathize with the protagonist at all.