I bought Monsters of the Multiverse this year, but I haven't really used it, so slightly regret it.
I was planning on buying Planescape but the amazing combo of wildly expensive + not actually having much in it (like, not even Bariaurs? Come on...) put me off.
Up until 2021 I'd bought almost every sourcebook (just skipping the Rick & Morty and Penny Arcade ones), but since then it's been like, it's not that the concepts/themes for most of the books are bad, but the way they've executed them, I don't want them. The exception re: concepts/themes was the Dragon and Giant books, I didn't inherently like those and the execution didn't change that. Settings have been downhill in quality and price/value since Theros. VRGtR, Spelljammer, and Planescape all suffered from just not enough actual "setting books" content, with VRGtR trying to do too much with too little space, Spelljammer just failing profoundly at being a setting book at all, and Planescape less bad but still missing stuff that seems like it would both be basic and very easy to do. Strixhaven and Dragonlance were adventures with a bit of setting on the side - and not even good as adventures (I can tell from critical reviews that Dragonlance would wind me and my main group up - again a pity, I was intending to get and run it).
I don't generally buy adventures, but WotC managed to really do an amazing job by screwing up the two times recently I was seriously considering it due to laziness - Dragonlance and The Shattered Obelisk.