Ones regarded as "classics" are a great filter. And there are so many at this point that you're unlikely to need many others. Like, you don't need 300 picture books [EDIT: Full disclosure, we probably do have 300 picture books, but we have a problem with books]. 50-80 is plenty and you can fill that many with classics, no problem, not even close to running out.

You can also just go flip through a few dozen at the library. You don't have to buy all your books, and you can use the library to find decent ones you might want to have around all the time. Also, just ask the librarians, like, "what's a good children's book with dinosaurs?" or whatever. Or look out for the ones they're highlighting with placement.

[FURTHER EDIT] Here's a new parents starter pack I guess, why not:

1) Suess — Hop on Pop, The Sleep Book, Horton Hears a Who!, I Had Trouble in Getting to Solla Sollew, The Lorax, et c. They almost all read smoothly despite all the gibberish (reading clunkily aloud is a real problem with books—I've had "easy" children's books that were hard to read aloud and "hard" books that were actually pretty easy to read aloud, like Woolf's To the Lighthouse [you can read literally anything to kids who are too young to focus well on pictures, or if they're in the bath or otherwise unable to really look at the book anyway—I'm no specialist in child development but I'd assume just hearing language-noises is all they really need, then; three-for-three remarkably-early talkers with sky-high working vocabularies before 18 months, so, dunno if that helped, but it must not have hurt, though I suspect it's mostly luck/genetics])

2) Madeline. Classic for a reason.

3) Wild About Books (reads great, lots of fun visuals; I had this damn thing memorized at one point, didn't even need to look at the text)

4) Alice in Wonderland and The Jungle Book are good read-alouds that aren't picture books.

5) How the Sun Was Brought Back to the Sky — good entry in the "quest with repetitive steps and a character picked up each time" genre, which is a whole thing in picture books. See also Room on the Broom, which is also quite good and reads well. In fact, maybe prefer that one.

6) Lots of the early Golden Books that aren't licensed properties—The Little Engine that Could, et c.

7) Mike Mulligan and his Steam Shovel

8) The Velveteen Rabbit (beware—it'll make the room a tad dusty)

9) Mother Goose nursery rhymes, enough exposure that they won't miss references to them, at least (I suspect we're losing these as a cultural touchstone, but enough references hang around from past media that they're good to be familiar with—I'd wager what's replacing them and a bunch of other classics is Disney shit, which has overwhelmed kids' media and brain-space). Popular collections of these are readily available used, they're everywhere.

10) D'Aulaires' Norse and Greek Myths books. Read-aloud early on as they're not picture books (but they have pictures), later they can make good books for the kids to read themselves, if they're into this stuff.

There are dozens to hundreds of other good ones, but those are a decent start. You'll probably regret few or none of those purchases.

[EDIT ONCE MORE] If you want older-school but not academic-level-of-old-school versions of fairy tales (good for reasons similar to Mother Goose, above) and don't require them to be picture books, just get the first couple of Lang's color-named Fairy Books—"Blue" and "Red" cover a lot of ground. Later spinoff entries are great if you want to familiarize kids with other oft-referenced-in-Western-culture stuff like saint and knight tales—The Red Romance Book, The Red Book of Heroes, The Book of Saints and Heroes, British-tinged folk and history tales in the two "True Story" books. It's a long series and some entries pull in tales from a wide variety of non-Western cultures—you won't exhaust this series in a kid's early childhood, most likely.

https://en.wikipedia.org/wiki/Lang%27s_Fairy_Books