What is Platos most famous or significant work?

Question

and what is it about?

Answer 1

"The Republic," arguably.

It is essentially a play about philosophy and politics. You can find the text of it here: http://ebooks.adelaide.edu.au/p/plato/p7...

"The Last Days of Socrates" is also popular. It is about the life (and death by hemlock) of Socrates. It defines what is known as the Socratic Method. The text of it is here:

http://socrates.clarke.edu/aplg0150.htm

Answer 2

The Last Days of Socrates because it is written by Plato, and tells of one of the other greatest philosophers of all time.