It may seem like this now, but I bet that this doesn't end up being true...I've not read all of his books, but I think his most underrated book, because it didn't go down well with everyone but I loved it and thought it completed a really really tough task, was The Gathering Storm.
His most overrated so far has been Alloy of Law, but I've liked them all. Not in any rush to read Warbreaker or Elantris though based on what I've heard.
I think his BEST book, that I've read, is Word of Radiance.