It's tough to come up with an all-time greatest achievement. But here are a few individual and team achievements that immediately come to mind: (1) Joe D's 56-game hitting streak. (2) Secretariat's Belmont domination. (3) Ali's defeat of the seemingly invincible Foreman. (4) The Miracle on Ice. (5) Lance Armstrong's Tour de France run post-cancer.
Gotta go with Lance Armstrong's Tour de France victories. Being a hoops fan, I think Oscar Robertson's season averaging a triple-double is just incredible as well.