The first film (A New Hope) was a trailblazing, although corny movie that was a lot of fun. The dialog is atrocious, but if you don't take it too seriously it's a blast as a film for kids. Alec Guinness's performance as Obi-Wan Kenobi together with James Earl Jones as the voice of the menacing and mysterious Darth Vader absolutely carries it. The robotics and costumes are very impressive and fun. Lucas ripped a lot of it off from film serials and Japanese cinema, but the universe he created and the way he blended everything is very creative and neat. Clearly relatively low budget. The second film (The Empire Strikes Back) is also quite good. More corny space fun, introducing the lovable Yoda with some great puppetry. The studio was willing to fund this one as much as Lucas wanted, and he made the most of it. Effects-wise, it's vastly superior to the first film, and is quite a bit darker in tone. Comparing the lightsaber duels from the first and second is the starkest contrast. Some people prefer it over the first. Introduces some neat elements. The third film (Return of the Jedi) is an extremely inconsistent mess that doesn't make much sense, but is required viewing to see how the trilogy ends. There are some very iconic moments in the the third film, and the ending feels quite satisfying as a bookend to the series.
I would recommend anybody that's interested in film history, science fiction, or special effects in general watch the first three.
As a whole, the prequel trilogy is... interesting. They are extremely slow and dialog-heavy, but obviously very big budget for their time. Much of the films feels more like space politics movies than adventure films. There are some extremely cool elements and battle scenes. At this point George Lucas had been divorced by his wife, Marcia Lucas, who had reined in a lot of his retardation by cleaning up his films script, editing, and material-wise. George had free rein, and he made sure to use it for better or worse. I (The Phantom Menace) is probably the worst of the bunch. Jar Jar Binks is one of the worst characters ever projected on the silver screen, but Liam Neeson does a solid job in his role as Qui-Gon Jinn. II and III are improvements over I, but still sort of a disappointment. The dialog is even worse than the original three, and there are a lot of "meme material" moments that you're probably familiar with. Anakin is a whiny little bitch and ruins a lot of the mystique of Darth Vader, though I think Hayden Christensen does well with what he's given. The love element is very poorly executed, and Natalie Portman has absolutely no chemistry with Christensen.
Only check out the prequel trilogy if you grew up with them or really, really liked the first three. My biggest gripe aside from the slowness is 'explaining' elements of the space wizard samurai Jedi Order that were better left as mysteries.
As for the Disney or extended universe stuff? I've never seen them myself, nor do I care to. I hear they're absolutely terrible. I wouldn't recommend anybody check them out.
Actually I did see Rogue One, which I would recommend if you really enjoyed the original three. Not nearly as good, but has some cool tie-ins and fills in some blanks.