who is the most famouse sculpture?

I don't see very clearly what you're asking. Do you want to know "who" is the most famous "sculptor" in the world, or "which piece of work" is the most famous work of "sculpture" in the world?
Moreover, "famous" is a very subject concept. It is impossible to say what is "most" famous.
Anyway, let see if the following information could be of any help.
"David", a masterpiece of sculpture by Michelangelo
----sculpted from 1501 to 1504, "David" is a masterpiece of Renaissance sculpture and one of Michelangelo's two greatest works of sculpture, along with the Pieta. It is the David alone that almost certainly holds the title of the most recognizable statue in the history of art. The 5.17 meter (17 ft) marble statue portrays the Biblicle King David at the moment that he decides to do battle with Goliath. It is now placed in Florence, Italy.
see: http://en.wikipedia.org/wiki/David_%28Michelangelo%29
Michelangelo, unsurpassed master of art
---Michelangelo, an Renaissance Italian painter, scultpor and arhitect, attained unsurpassed achievement in art. The best of his two scultpure works, David and Pieta, are recognised as defning Renaissance art of sculpture.
see: http://en.wikipedia.org/wiki/Michelangelo

Sculpture: a work of art that is solid figure or object made by carving or shaping of wood, stone, clay, metal, etc.