1. Episode IV: A New Hope (1977)
2. Episode V: The Empire Strikes Back (1980)
3. Episode VI: Return of the Jedi (1983)
This is the order in which the films were released and the order in which the story unfolds chronologically.
While the prequel trilogy (Episodes I-III) was released later, it tells the story of events that happened before the original trilogy.
Therefore, for a complete understanding of the Star Wars saga, the order you should watch the films is:
1. Episode I: Phantom Menace (1999)
2. Episode II: Attack of the Clones (2002)
3. Episode III: Revenge of the Sith (2005)
4. Episode IV: A New Hope (1977)
5. Episode V: The Empire Strikes Back (1980)
6. Episode VI: Return of the Jedi (1983)