The greatest chess move of all time? Easy. It's when black after 1.e4 plays 1...c5.
You don't agree? Well, I don't think you are alone :)
Then I have instead another candidate for the title: a move played in the game
Veselin Topalov - Alexei Shirov, Linares 1998.
Position in the game after 47.Kh1-g1.
The game at Chessgames.com
Analysis by Mark Dvoretsky (page 2-3) at ChessCafe.com (pdf)
A video at YouTube
See also at Chessgames.com the game Alexei Shirov - Ulf Andersson, Biel 1991
You don't agree? Well, I don't think you are alone :)
Then I have instead another candidate for the title: a move played in the game
Veselin Topalov - Alexei Shirov, Linares 1998.
Position in the game after 47.Kh1-g1.
The game at Chessgames.com
Analysis by Mark Dvoretsky (page 2-3) at ChessCafe.com (pdf)
A video at YouTube
See also at Chessgames.com the game Alexei Shirov - Ulf Andersson, Biel 1991