I highly recommend this order.
Episode 4 A New Hope,
Episode 5 The Empire Strikes Back,
Episode 1 The Phantom Menace,
Episode 2 Attack of The Clones,
Episode 3 Revenge of the Sith,
Episode 6 Return of the Jedi,
Episode 7 The Force Awakens.
That way you get the original story & surprise first, then the back story, then the conclusion. Makes the story of Anakin Skywalker much more powerful.
I also STRONGLY recommend The Clone Wars Movie (2008) and Series Seasons 1-6, if you don't want to watch all 6 years, and just want a general idea of what the clone wars were, The Arc I suggest to start with is in season 3; The Mortis Trilogy. These 3 episodes were GAME CHANGERS, and everything that happens in them and after them are very relevant to the story. It's weird watching Star Wars as Animation at first, but after a while, I forgot it was a "Cartoon" and it just felt like normal star wars to me. Rebels is good as well, the current animated show, but it is not as significant to the overall trilogy as the clone wars, most of the characters on rebels are original just to that series, except for a few that tie into the clone wars series, more than the movies themselves.
For reference, The Clone Wars timeline is in between Attack of The Clones and Revenge of The Sith, and Rebels is in between Revenge of the Sith and A New Hope, closer to A New Hope Era.
