1. Revenge Of The Sith
2. Empire Strikes Back
3. The Phantom Menace
4. Attack of The Clones
5. Return Of The Jedi
6. A New Hope
But in a sense, I like each movie equally. I have the same amount of Pros (give or take a few for certain films) as I do Cons. To bash 1 film in the STAR WARS Universe is pretty low, you either love STAR WARS, or you hate it. None of this inbetween "omg I can only watch Episodes 4 - 6" or "OMG I Can only watch Episodes 1 - 3" but as the above poster said, he likes episodes 3 - 5.
If I had to pick 3 to show somebody first, I would pick Episodes 1 - 3.