The time when WotC has the only Monster Manual-style core monster book is already over and will get less over as time goes on, post SRD going into Creative Commons.
We already have Level Up's
Monstrous Menagerie. Kobold Press is crowdfunding its
Monster Vault now. Presumably we'll see something similar from Cubicle 7. And they're not likely to be the end of such books. And, of course, there's the 2024 Monster Manual, which will presumably look a lot like Monsters of the Multiverse.
Some folks, with big wallets and expansive shelf space, will likely get all of them. But if you were choosing
just one, how would you decide?
Well, I only have 2 monsters books of this edition, the MM and VGtM.
The MM has been widely criticized (by me as well, back at the beginning of 5e) for failing to deliver enough monsters abilities and variants, as were promised during the DnDNext playtests. We got way too many monsters with a single special ability (some, not even that) and very few monster variants (mainly some 'boss' versions of humanoids).
But through the years, I've learned to see 5e as a toolbox rather than a static set of material. I don't care to buy another monsters book, when I can very easily take any MM monsters, and create variants by myself, adding abilities taken from other monsters, adding feats, or even adding class levels (whatever I need from them, with no regards to 'rules' for adding classes to monsters, which in fact don't even exist), and then ballparking the resulting CR/XP, or just don't even give a flumph about it... drop the monster in combat and let the PCs deal with it one way or the other.
Maybe, if the designers were really able to come up with innovative new abilities for monsters, I might consider other books. But what I see most of the times (and not just with monsters, the same also happens with PC's class archetypes) is hardly innovative at all, just colorwashed old abilities, with few exceptions.
So there it is, the MM as the only monster I will ever really need, to set the starting point of a few hundreds famous monsters across the CR spectrum.
If anything, I'd rather need more
monsters stories, not more stats. But for that, I have 40 years of D&D history before 5e to look at, plus thousands of non-D&D books (and non-books) as well.