I would suggest watching the Star Wars Saga in the following order.....
1. Episode 4: A New Hope
2. Episode 5: The Empire Strikes Back
3. Episode 6: Return of The Jedi
At this point you should be convinced that you have just seen the greatest movies ever...Here comes the suck....
4. Episode 1: The Phantom Menace ( An awful movie, you'll think this is as bad as it gets.....but hold on)
5. Episode 2: Attack Of The Clones ( George Lucas has lost the plot......Thankfully the low point of the series.)
For the full experience you should also consider watching the Clone Wars cartoon series which is about 100 odd episodes.
These should be viewed between episodes 2 and 3. The Clone Wars are brilliant!!!
6. Episode 3: Revenge of the Sith (The best of the prequel trilogy, which isn't saying much if I'm being honest.)
Finally you should watch the Star Wars Rebels cartoon series which is currently being shown on Disney XD. Series one was a bit slow but series 2 is only half way through and it is absolutely brilliant.
This will bring you full circle back to Episode 4.
I could also provide a list of books, comics etc that should enrich your experience if you would like.