Kasparov
All Time Greatest
I'd second Sultan Khan. To be unaware of the international rules of the game (he played chess according to Indian rules which are vastly different), not literate, and the best hope of getting any information was from people who would be his opponents - and as yet, a grandmaster-strength player who even bested Capa once! Such a phenomenon has never been seen before, or since.
Of course, my other choice would be Paul Morphy. If it took the great Steinitz 15 years of effort to unravel the ideas behind Morphy's games - that ought to be sufficient indication of how far advanced a player Morphy was.
Who do you think is the greatest chess player of all time ?