In the spirit of the topic, I'll offer Smyslov as the most under-rated world champion.
When you read the unfettered praise for Smyslov from the likes of Fischer, Spassky, Tal, Kramnik, and Kasparov, you realize Smyslov was a rare and special genius of 20th century chess.
He didn't have a aparkling style like Tal or a fierce competitive drive like Fischer or Kasparov, but he had something. Yet go to Amazon and see how few books on him are in print.
Because of the apparent simplicity of his stye, Smyslov is rarely mentioned among the players who have made the greatest contribution to the development of the ancient game. However, his victories at the peak of his career are amazing for the lack of a clear defence for his opponents, and a careful study reveals that no one in the world could withstand Smyslov's very fine technque.
--Kasparov
How would you describe the seventh World Champion, Vasily Smyslov?
How can I express it in the right way? ... He is truth in chess! Smyslov plays correctly, truthfully and has a natural style. By the way, why do you think he lacks that aura of mystique like Tal or Capablanca? Because Smyslov is not an actor in chess, his play is neither artistic nor fascinating. But I am fond of his style. I would recommend a study of Smyslov's games to children who want to know how to play chess because he plays the game how it should be played: his style is the closest to some sort of 'virtual truth' in chess. He always tried to make the strongest move in each position. He has surpassed many other of the World Champions in the number of strongest moves made. As a professional, this skill impresses me. I know that spectators are more interested in flaws ... ups and downs. But from the professional standpoint, Smyslov has been underestimated.
--Kramnik Interview
Most Underrated Chess Player of All Time
Mir Sultan Kahn
Yasser Seirawan
Carl Schlechter
Borislav Ivanov is clearly the most underrated player of modern times.
That drunk gm who lost cause he was napping. To paraphrase Willie Nelson, I probably couldn't find the board if I was drunk
chessmaster series is the most underrated programme. Can a gM win against a Chessmaster in a best of 10 games?
Maróczy and Staunton. Maybe also Leonid Stein.
Mikhail Chigorin. One of the first five grandmaster and he paved the way for all the Russian greats after him.
I'd submit Stepan Michailovitsch Levitski and Frederick Lokes Slous as two highly underrated players.
Chigorin, Mason, Stein, Janovsky
Rezsö (Rudolf) CHAROUSEK!
Per favore, cerca di essere pertinente, utile e gentile!
Fine, Pillsbury, Boleslavsky, Harrwitz.
Areez. (Me)
What about the Rev John Owen? : http://www.chessgames.com/player/john_owen.html
He played a famous positional mastrepiece once that was cited by Chernev as being an example of "broad design". I can't recall the game - if someone can please tell me. He played the top players of his day in a long career.
The most underrated chess player has got to be... . Darn it, I forgot his name.
Definitely Max Lange, the Tal of the 19th century.
Larsen is mainly remembered for that match against Fischer, but just play through for example both his games against reigning World Champion Petrosian in Piatigorsky Cup 1966... One positional and strategical masterpiece, and one crushing kingside attack. Two great wins against a very hard to beat opponent, that show what a player Larsen was at his best.
Carlsen is rated, not under or over.