I would recommend to start with 1 (the last one of the newest movies) and then work your way up to 6, that way you have the timeline in chronological order. It does also mean that you start with the coolest effects first, but the 1st three movies (Part 4 - 6) are still surprisingly good.
The order I would watch them in is:
Star Wars I - The Phantom Menace
Star Wars II - Attack of the Clones
Star Wars III - Revenge of the Sith
Star Wars IV - A New Hope
Star Wars V - The Empire Strikes Back
Star Wars VI - Return of the Jedi
Enjoy the movies!