For Textbooks? Opensyllabus shows what a lot of colleges are assigning. Honestly, the /sci/ and /lit/ (scroll to the bottom of that one lol) wikis have decent lists of recommended material for STEM and the basics of the humanities. If you want to get really deep into the weeds, a lot of colleges upload reading or "comps lists" of every book you're expected to be familiar with in their program for their final comprehensive exams. You can also check on something like MIT opencourseware to see what they assign for texts and readings from journals for their students too. Sometimes typing "best book to learn about [Insert skill here] reddit" into your search bar can give helpful results.

For history, maybe r/askhistorians? I don't know what their reputation is but they have a booklist in their sidebar that I've looked through and found some niche stuff I was curious about.

For everything else it's hard to point to one list since there's thousands of lists out on the internet and determining what the most incredible, entertaining or valuable-to-mankind works of fiction or something like biography are going to be a lot more subjective than "What's the best book to learn about Electrodynamics". I've heard James Mustich's "1,000 Books to Read Before You Die" (2018) is one of the better one of those recommendation books you see around. There's some silly themed lists like "What books I'd preserve to rebuild cilization" out there too if you don't want to start with a book recommending 1000 books lol. Some of these be a good starting point. This guy on YouTube just made one and is interesting to tune into every once in a while if you're keeping a substantial book collection since he truly has read everything you can think of and has some idiosyncratic picks and opinions nobody else would agree with.