Paul Keres. The best player to never be World Champion.
That doesn't sound like he was underrated. Of course, Viktor Korchnoi and Magnus Carlsen would probably dispute your claim anyway.
Among world champions, Max Euwe is underrated. He was still probably the weakest of those chess gods who became match-play champions, but he was definitely deserving.
Aron Nimzovich is often overlooked as a great player in his own right, yet he was probably one of the top three players in the world in the decade after World War One.
Do we get to have a woman and male catagory? Susan Polgar asked the same question about the most underrated player: http://susanpolgar.blogspot.co.uk/2008/02/most-underrated-players-ever.html
No, I am afraid we don't. I am so sorry.