The Empire Strikes Back (1980) - The Unexpected Sequel: Fantastic sequel but an aberration. Far darker in tone and hints at maturity but don’t ever hope for that mature tone to ever come back in any way again.

Star Wars (1977) - The Unexpected Hit: If you don’t see any of the other films, you should watch this. Forget the ideas of backstory and such, this is the distillation of the best of George Lucas’s ideas.

The Force Awakens (2015) - Reviving the Franchise to Milk it Some More: This is not a bad film, but you won’t get much of anything new. It’s whole reason to exist is to establish that the Star Wars franchise has new owners and they are here to give you exactly what you want with nary a hint of edge, maturity or growth.

The Prequels (1999-2005) - The Milking of the Franchise as a Special Effects Demo Reel: Nothing shown on screen here—or explained in dialogue—is worth remembering. The films mainly flesh out backstory that doesn’t need to be fleshed out and primarily acts as special effects house demo reel.

While there are many good answers here—such as Nick Bedford’s simple and succinct list—that explain differing rationale on this topic, I believe nobody is really addressing the value of simply watching them purely in production/release order with a contextual understanding of why each film was made. So here I go: My general feeling is the films should be watched in the order they were released and not “episode” order; the story is not that deep and the “revelations” are not that revealing.

Star Wars (1977) - The Unexpected Hit: Honestly, you can just watch this one film and never watch anything else and be happy. This film is 100% self-contained and pretty much a perfect distillation of the story George Lucas has attempted to tell without wasting time on—frankly—the tons of extraneous backstory nonsense even hardcore fans. There is a clear beginning, middle and end and the backstory elements serve their purpose; they are added depth that allows a fairly simply space fairy tale to be told.

The Empire Strikes Back (1980) - The Unexpected Sequel: Great sequel, but an oddball none-the-less. When this was being produced it’s working title was simply Star Wars II; not “Episode V” or some nonsense like that. As good as this film is—and it’s a very good film—it was/is still only a film that was made simply because the first film was a runaway hit. The demand for a “sequel” was mainly to rake in more of that sweet, sweet money the first film brought in. That said, this film is an oddball in the whole series simple because it is an unexpected sequel; utterly nobody involved in the production could conceive at the time that a sequel to Star Wars would be successful financially or creatively. So what you have here is an attempt to deepen the threadbare plot of the first film. And that was done by expanding character depth in many different ways. Where the first Star Wars played out as a children’s film that adults could enjoy, The Empire Strikes Back feels like a young adult novel with far more true coming of age and series tones taken. The biggest of which is the film ended not with anything positive, but a fairly simple, “And these are the struggles of our heroes…” footing. But still, you need to see this film as an aberration to the Star Wars vision; it was/is an experiment to see what a sequel to a blockbuster such as Star Wars could be but in many ways was too dark for George Lucas’s tastes.

Return of the Jedi (1983) - The Creation of a Franchise: If Star Wars was an unexpected hit, and The Empire Strikes Back was an unexpectedly successful sequel, this film could be seen as George Lucas now defining not what a sequel could be, but was a franchise should be. With the success of those two films, it was clear that if the films stuck to a specific formula, the series could simply be a proverbial goose that just always laid golden eggs. And that is what you see on screen here. Gone were even plot explanations and character rationale, and instead we have characters being played in a “winking” manner with tons of goofy side characters such as Sy Snootles and such. And after setting up Boba Fett as a truly scary “who is that guy” character in The Empire Strikes Back, what do you get in this film? They just kill him off in a fairly pathetic way that’s punctuated with the huge monster who consumed him just burping? Also, as many people deride The Force Awakens (2015) as being derivative of the 1977 film, this film is pretty derivative of that first film as well; I mean the whole plot past rescuing Han Solo was to blow up another Death Star. That said, this film is not entirely bad and there are some very cool action scenes—the speeder bike chase I think is fantastic—but it’s truly the moment you realize any semblance of young-adult to adult depth the series might have had was tossed out the window. To quote Phil Tippett—the stop-motion artist for the original “Star Wars” trilogy—George Lucas explicitly told Return of the Jedi director Richard Marquand, “Well, what we’re doing now is kind of like a cross between Benji and what we did on Empire Strikes Back.” This is the film when George Lucas decided pandering to the audience was better than respecting the audience’s intelligence and growing the story in a mature way.

The Prequels (1999-2005) - The Milking of the Franchise as a Special Effects Demo Reel: The milking of a franchise. In my humble opinion, you never have to watch any of this stuff for any reason; not for plot and not even for action sequences. The way I see this stuff is simple: All of what is presented here is just backstory to characters George Lucas created for the 1977 Star Wars film. You know what backstory is? It’s nothing special; it’s just the simple rationale and ideas a creative person would sketch out as subordinate to a greater story. Meaning there are backstories to practically every character in practically every work of creative fiction that exists out there; it’s a normal byproduct of a creative process. But honestly, by byproducts—in and of themselves—are not worth dealing with. If anything these prequels were produced simply to show off the special effects magic that ILM (Industrial Light and Magic) can do. So in that way, these films act as very expensive and elaborate demo reels of the high quality work ILM can create. Which—on their own—look cool but are ultimately devoid of any real plot where you actually care about any of the characters. These films are just hollow and forgettable.