If you only want single book entries, you probably need to go back to classic literature.
Anything that's good will be the victim of its own success and get more installments.

The Hobbit.

Anathem by Neal Stephenson

This is a collection of books. However, if I'm remembering correctly, each book in the collection can be read separately. They do not depend on the previous entries.

Whenever anyone asks which book setting I wish I could live in and explore, this is my answer: The Culture, by Ian M. Banks. Each book describes a fascinating universe with complex politics and relatively unique powers/characters.

The Priory of the Orange Tree by Samantha Shannon fits here.

While it does technically have a sequel (which is a prequel, if I remember correctly) set in the same universe, the first book is a complete story unto itself.

The world feels vast and detailed, and the characters are satisfying. Samantha Shannon's prose is great as well. Highly enjoyable book!

The Dosadi Experiment by Frank Herbert. There was a previous book called Whipping Star that sets up the universe, but it's not necessary to enjoy Dosadi.

It’s really hard to compare (nearly complete) series with a standalone book.

Maybe try Mistborn: The Final Empire, and just read that specific entry of the Mistborn series and ignore all the other Cosmere books.

Unfortunately I’m not very good an answering this question, because I often go for long running fantasy series and not one off books.

I’m assuming you’re looking for something in the realm of high fantasy, right? Or are you genre-flexible?

Clive Barker's books are good for this. If you want fantasy in particular, try Imajica and Weaveworld.

River of Gods by Ian McDonald.

And probably his other books also, but I haven't read them yet.

Okay it's 3 books but they're short and it has an ending lol

The silo series (wool, shift, dust)

Also I would check out the Mistborn series. It's more books, but many of them (like mistborn) can stand alone and be a great story. Insanely good world building, some of the most unique out there I think

The Prince of Nothing is a great series and pretty complex.

In single standalone book? That's a bit of challenge. Try Dune book 1, Brandon Sanderson Elantris and Warbreaker, Henryk Sienkiewicz The Knights of the Cross and Trilogy (Trilogy can be read as 3 separate books though, it's tied more by general topic and historical period than by any plot). Also Martin's Fire and Blood, it's finished and the writing style is different than ASoIaF.