Nimzovitch, Korchnoï, Keres, Bronstein, Rubinstein, Topalov, Ivanchuk, Rehevsky, Pillsburry, Larsen
The top 10 most famous chess players that never became world champion

Borislav Ivanov,he became famous for his cheating capabilities and has acquired cult-status amongst cheaters.
Do we know that he cheated? Confession or evidence? Has anybody found the device/cheatmachine?

Borislav Ivanov,he became famous for his cheating capabilities and has acquired cult-status amongst cheaters.
Do we know that he cheated? Confession or evidence? Has anybody found the device/cheatmachine?
No we don't know for sure.But the fact that he refused to have his shoes searched combined with his way-above-his-elo performances make it highly unlikely that he was not cheating..

I agree with lots of names already discussed, so I thought I'd throw in some other players that might have deserved a mention:
Rashid Nezhmetdinov, never became GM, but scared most soviet players (including Tal, Spassky, Geller, Bronstein, with the exception of defenders, e.g. Petrosian)...
Luke McShane, considering he held a full-time job during most of his chess career.
Yoshiharu Habu (羽生善治): consider what he could have done to chess players if he were not busy becoming one of the strongest shogi players in history.

I want to do something out of the ordinary and elect Tal and Spassky's trainer and second Koblents for his wonderful contributions to chess; and yes he was a master.
When did Koblents work with Spassky ? He may have been a second to Spassky at some point but Spassky only had 3 trainers : Zak , Tolush and Bondarevsky , that I am aware of ...
Borislav Ivanov,he became famous for his cheating capabilities and has acquired cult-status amongst cheaters.