The prequels should all be non-canon and almost every piece of EU was better written
The prequels were a good story badly told and that lies mostly at the feet of George Lucas being an all-sizzle, no-steak writer who has basically admitted that he sees dialogue as something more like an inconvenience necessary to get you to the next action sequence than something integral to the story. The story concept was good, but the execution was terrible because every scene is underpinned by people sounding like no human being (or humanoid alien) should ever sound like while delivering their lines. Harrison Ford was right when he snapped at Lucas during the first movie and said "you can write this shit, George, but you can't
say it."
Even the Phantom Menace and the basic idea of them "randomly" discovering (arguably through the will of the force) Anakin on some remote backwater world and finding out that he is prodigeously talented with the force even if he doesn't know it and then having him somewhat involved in their escape from there since Palpatine is hunting them and the war has been brought to them on Naboo is totally fine in theory but ruined because everyone sounds like an idiot and every bit of dialogue-driven narrative progress is delivered with all the smoothness of riding a car with no suspension down an old gravel road. The Clone Wars show proved that capable writers can deliver a quality story in that setting when given the opportunity. Quite honestly it is the single best thing out of Star Wars fiction because it takes the good ideas inherent in the prequel trilogy and makes them work. Even the political stuff works because it actually gives Padme something to do that plays to her strengths instead of Episode II's super clumsy action hero gladiator pit stuff. She isn't a jedi. She's smart, cultured, savvy, and understands the political machine and how to use it. And that's where she shines while the Jedi would be reduced to just swinging their swords around.
As for "almost every piece of the EU was better written" I don't know. I haven't read everything and I am trying to start right now in and around the original trilogy before I work backwards and forwards from there. But from what I understand things get rough as we get into the books published in the early-mid 2000s and beyond (until the Disney buyout) things get a bit ropey and divisive.
Honestly if we set aside prequel era books for people that don't want to be tied to the story sprawled out from those movies since they almost all come after the movies were released with the exception of the handful of books that happen immediately before the original movies, the only things I see get universal praise are everything written by Timothy Zahn (but especially the like 6 books he wrote pre-Disney that focus around Thrawn and that story), AC Crispin's Han Solo trilogy (separate from Brian Daley's Solo trilogy from the 80s), The X-Wing series, and the "Tales from..." anthology books. New Jedi Order is divisive, The final two series of books are divisive, and the like 7 books that take place after the Jedi Academy Trilogy and its pseudo-mate book "I, Jedi" all suck to varying degrees. I have had the hardcover of Children of the Jedi for like 25 years and have basically never touched it because I've only heard how terrible it was and that its writer is considered one of the weaker ones among the multiple-book EU authors.
I tried to look up any analysis of the whole Legends/EU run for some sort of consensus and the closest I can find is a series of really clumsily done blog posts about collated 5-star ratings from various websites that spends more time talking about the math functions used than really discussing what any of it means, and which in the end clumps basically everything between average ratings of like 4.3 and 3.4 with only one series (the infamous "Jedi Prince" kids' novels) falling below 3.0. So it's hard to say that there's any broad agreement on where things are good or bad because all you really learn is that given a wide enough audience most stuff is going to fall into the mushy range of "fine."
That's the reason I'm so taken aback by Luke referring to The Emperor as "Darth Sidious" in The Last Jedi
We came SO close to the Disney trilogy avoiding any prequel references and that moment dropped the ball
It then also opened the floodgates for. . . whatever the HELL The Rise of Skywalker was, that f***ing mess
I could almost excuse Luke for using the name if it was used in concert with calling him Emperor Palpatine to demonstrate that Luke has put all the pieces together. But instead it felt clumsy and like it was made under the mistaken impression that super-fans would prefer his "true sith name" rather than the one that was more heavily used throughout the previous two eras. I don't mind if they want to drop prequel references because there are going to be people that grew up on those movies and look past the flaws in them too just in the same way that all of us that grew up on the originals look past their flaws (spoilers: George Lucas' shit writing was still present in the first 3 movies. It was just a bit blunted by the fact that he had other writers and editors on board to sand off his rough edges)
The sequel trilogy's problem is entirely down to the fact that The Force Awakens was basically a re-do of the original movie in a way that didn't properly set up for a sprawling new era of Star Wars to explore, then all the behind-the-scenes drama meant that we basically got 3 competing visions across all 3 movies because you had Abrams' original setup, Rian Johnson trying desperately to throw most of that away in The Last Jedi and running out of space to tell a good story in Ep 8, and then Abrams coming back and desperately trying to frankenstein together a clumsy mess out of the remnants of his original ideas, whatever bits of Johnson's work he didn't feel like discarding, and a bunch of what felt like lazy decisions just because he clearly wasn't ever going to enjoy crafting a movie that wasn't what he planned to do to begin with. Nothing worked because every movie worked against the others.
I thought TFA was alright, but it needed its sequel to be its Empire Strikes Back in order to better solidify it and make it worthwhile. Instead we got a trash heap that made basically everyone unimportant except Rey and Kylo Ren and then went f***ing nowhere with anything they set up (Rey being literally nobody would've been a fun twist. Kylo Ren trying and failing to redeem himself and being the ultimate villain would've been cool and tragic. Luke being bitter because he failed is fine (although I think his actual failure was pretty dumb in and of itself), but the way he went out was stupid. etc)
I'm going to have to make my peace with the sequel movies eventually just like I did with the prequels. I'm at least at the point now where I can watch the prequels and enjoy the parts I like about them for what they are (though the Phantom Menace less so). But part of my drive to get back into the Star Wars books was to go through the alternate take of franchise history and the alternate after-story that doesn't just discard everyone and spin its wheels to tell a clunkier version of hte original trilogy for no good reason.