Tolkien!
If you've not already read him, of course. The Lord of the Rings is almost standard reading for people by this point. If you haven't, though, then I would warn you that his style of prose is definitely not for everyone, as he's very long-winded about lore and descriptions and it's all a bit old-timey. But Middle Earth is one of the best-realized fictional universes I could name. If you have read The Hobbit and The Lord of the Rings, then why not expand to the posthumous Middle Earth works that Christopher Tolkien has edited together over the past forty years? Unfinished Tales, The Silmarillion, The Histories of Middle Earth, The Children of Hurin, etc. etc., they're all a bit less focused than The Lord of the Rings because they're even more concerned with lore and backstory, but as a way of fleshing out the world it's pretty top-tier.
The Ultimate Hitchhiker's Guide to the Galaxy is one I would recommend for a bit of a more light-hearted jaunt, if you've not read it. It collects Douglas Adams' five Hitchhiker's Guide books into one volume (as the books themselves get shorter and shorter as they go along). It's comedic sci-fi, and pretty good at it too.
As far as longer fantasy is concerned, a series which frequently comes up in these recommendation threads is Brandon Sanderson's Mistborn trilogy. It's widely lauded for Sanderson's ability to really craft his world and its use of magic and other supernatural elements. Then there's the Gentlemen Bastards series by Scott Lynch, which currently has three books released (The Lies of Locke Lamora, Red Seas Under Red Skies, and The Republic of Thieves) and is about a group of "elite con artists", to use Wikipedia's phrasing. Also Patrick Rothfuss' The Name of the Wind and The Wise Man's Fear, the two books currently released in his The Kingkiller Chronicle trilogy.
While they are much shorter, I'd also recommend the works of Terry Pratchett and Neil Gaiman. A good entry point to both of them might be Good Omens, which they co-wrote together. Pratchett is well-known for the comedic fantasy Discworld franchise, which is very large and spans many, many novels which often follow separate narrative paths so it's a bit complicated to suggest a good starting book, while Gaiman is known for more dark and weird things (the comic-book series The Sandman and the book upon which the film Coraline was based might be his two best-known works; American Gods, Anansi Boys, Neverwhere, and Stardust are my recommendations for him).
Something a bit more classical, if you're at all interested in Arthurian legend then check out T. H. White's The Once and Future King, which A Song of Ice & Fire has often been compared to and is probably the go-to example of mature literature centered around King Arthur. And for classic sci-fi, you can't really go wrong with Robert Heinlein's Stranger in a Strange Land or Frank Herbert's Dune.
And... well, that's a lot of books and would likely cost a lot of money already, so I'll leave it there I think.
