I, IV, II, V, (TCW), III, VI, VII, VIII, IX, (Rogue One, Solo, Rebels, Resistance, The Mandalorian, Cassian Andor, Kenobi etc.)

I am mostly joking - this would be very confusing for children - but I think this order does have a few interesting characteristics:

It starts the saga with TPM, which is (political plot notwithstanding) the most childish film in the saga, what with the literal children main characters, the poop jokes and the pod race, so it feels like an innocent starting place. It also introduces Qui Gon, who really encapsulates the concept of a Jedi. We see the glory days of the Republic and the Jedi, and are introduced to Anakin and Obi Wan, and learn that Obi Wan trains Anakin.
Then, IV. We learn from the crawl there is a civil war occurring - clearly things have changed since TPM, and we don’t recognise any of the characters except the robots. We see Luke on Tatooine, and perhaps for a moment you might mistake him for a grown up Anakin. It isn’t until we meet Obi-Wan “Ben” Kenobi that we learn that the Jedi are gone and the galaxy has fallen into darkness since TPM. A hypothetical first time audience might wonder what happened to Anakin, and possibly piece together that he was Luke’s father who Kenobi talks about, who was apparently killed by another pupil of Kenobi’s, Darth Vader. They’d certainly be curious about what happened to the Republic, and I think it adds a compelling element of mystery to Vader and the Empire and makes the few mentions of the past we get in ANH more meaningful.
Having learned the Republic will collapse, we go back to find out how, and what happened to Anakin. The political plot leading up to the Clone Wars is a little more interesting - perhaps we think that the separatists might overthrow the Republic and become the Empire or something. Keeps your brain slightly more engaged in the film’s mystery plot, potentially. Maybe we’re a little more invested in the romance, suspecting it will produce Luke. We see the darkness in Anakin, but no introduction of any character called Vader…
Back to the dark future with V. Having just seen II, you’d probably notice some of the fun parallels Lucas used, like the romance and the asteroid field and the jetpack bounty hunter and the limb loss in the duel. Of course, the twist that Vader is Anakin has been preserved, and is hopefully even more powerful now we know who Anakin is (especially if you’re watching a fanedit of II which makes him actually likeable lol). We also find out that Vader’s master is… Sidious, who we saw collaborating with Dooku at the end of II! Of course, anyone with a good eye for chins knows he’s also the Chancellor.
We know Anakin is Vader, and are probably curious as to how this has occurred. I recommend watching TCW here because of the great work it does for Anakin’s character and for building audience investment in the Jedi Order ahead of Order 66, which make III hit harder. Obviously it’s a TV series so it’s not feasible to slot it into a film marathon, but I’m working on a 3 to 4 hour edit focused on Anakin to serve this purpose when I test this order on myself in December (tentatively titled “Hero of the Republic”).
III solves the mysteries. We see how the Republic falls, how Anakin falls, Padme dies, Obi Wan becomes Ben, Yoda goes into exile… We also get the reveal that Luke has a twin sister: Leia! I think this reveal works much better in III than VI (unlike the Darth Anakin twist) because a hypothetical first time audience would not be expecting a second baby. Whereas in VI it’s just heavy-handed Dagobah ghost exposition.
We’ve hit the emotional low point of the saga. Now it’s time to get our satisfying, hopeful ending with VI. We’ve seen Anakin fall to the dark side, and perhaps fear that Luke will do the same to save his friends… But then we see Luke as a great Jedi, and really feel it when he says “like my father before me”, remembering the hero of the Clone Wars that Vader once was. Sidious, who we saw gradually gathering power through the three prequel films, reappears in full Sith Lord style from III to face Luke. Will Luke succeed where his father (and Yoda) failed? Anakin, who we’re as invested in as possible, is redeemed. The saga has its happy ending, and we see Anakin and Obi Wan reunited after death. The interleaved stories of Father and Son are resolved.
VII, VIII, IX: The PT and OT work well as standalones (or imo, interleaved) because they were produced non-chronologically and thus couldn’t rely on knowledge of the PT in the OT. On the other hand, you can’t watch VII without having seen VI because it requires knowledge of Leia and Han’s relationship, and Leia being Luke’s sister, in order to work out how Ben Solo is Anakin Skywalker’s grandson. And VIII is VII Part 2 so you can’t sandwich anything in between there. Obviously I haven’t seen IX yet but I doubt it’ll make it any more feasible. So just watch them as a trilogy; an epilogue or a “Legacy” trilogy perhaps.
Anthology films and other TV shows: only TCW focuses on characters from the movies, while Rebels, Resistance, Rogue One and so on all introduce new characters, so squeezing them into the saga interrupts the flow even more. Solo is very, very standalone and, even though I hope it gets a Disney+ sequel, it’s hard to argue it adds much to the overall saga (except for setting up Han’s dice). Regardless of chronological positioning, I think it’s best to watch all these afterwards.
Downsides: Yoda is revealed in I, not V, so you don’t get to share in Luke’s shock when he discovers the little frog man is the “great warrior” he is searching for. But perhaps we can laugh alongside Yoda instead. It also doesn’t fix weirdness like Obi Wan calling Vader “Darth”, or Vader claiming Obi Wan once thought he could be redeemed, even though that doesn’t really seem like the case in III. It is also somewhat confusing to watch two timelines. There’s probably other things I forgot.