There's an awful lot of material i haven't bought so can't judge. Strahd and Dragonlance are the only adventures i own, I never bought Ravnica or any of the adventure compilations, or Acquisitions Incorporated or Bigby's - so i can't really easily judge the whole line.
Of the stuff I do own - VRGtR gets worse every time i read it, but there's still enough legitimately good stuff in there to keep it off the bottom. SCAG isn't BAD exactly, just phoned-in and perfunctory. Xanathar's and Tasha's are both essential in my opinion, though the puzzles section of Tasha's is an insult to paper and Xanathar's missed a few big opportunities to fix inadequate stuff like the Mastermind, Inquisitive etc and the attempt to fix Pact of the Blade by making it viable for Hexblades and only Hexblades was dumb.
It probably comes down to Spelljammer or Strixhaven in the end.
Spelljammer is a space pirate setting lacking not only a whole lot of essential space piracy material, but also lacking actual setting, which is kinda a big deal for a setting book. Add to that a railroaded and uninspired adventure, terrible proofreading and quality control (not just the hadozee art, but also the hadozee glide rules which were errata-ed within weeks of the thing coming out but which EVERYONE warned WotC about in the UA survey), terrible value for money, a DM screen nobody asked for or wanted, and a monster book of which 1/3 of the pages are a big middle finger to Dark Sun fans, rubbing their noses in the fact that WotC will never ever do a 5e edition of that setting and stealing all the DS monsters for use in a completely thematically inappropriate different setting instead.
Strixhaven was just a case of ... why? I can understand the idea of a magic college setting - it's even an interesting idea to have a setting that limits classes to ONLY the magic users (even if WotC were too timid and conservative to do that in the end). But the book has almost no support for anything like running a college setting where things like, y'know, classes and exams and so on matter. The whole interpersonal relationships rules were a nice concept, but so lazily implemented as to be worthless (and ferociously min-max-able by polyamorous PCs!). And the adventure is annoying and repetitive and the stakes just seem ... silly and trivial a lot of the time. The whole thing and place just lacks - focus, theme, dynamism, a reason for being. What IS Strixhaven, and what do you do there if you're not running the cruddy low-level adventure? It just fails to provide any hooks or give me a reason to care. It's not like there's no compelling fictional magic schools. Aside from Hogwarts, there's of course Brakebills, or Naomi Novik's Scholomance just off the top of my head.
So yea, one of those two.