It's probably a toss up between the Discworld series and the Bobiverse series.

Edit: for standalone book it would probably be Project Hail Mary.

Shameless plug:
lemmy.world/c/bobiverse

Hitchhikers Guide series read by Adams himself. His voice is just so good. Stephen Fry a close second.

Children On Time trilogy. Together with Three Body Problem generally the best Scifi I've read or listened to.

The Lord of the Rings series read by Rob Inglis is awesome. That’s probably my favorite way to consume the series.

World War Z. Because of the cast.

The Hyperion books by Dan Simmons. Hyperion has full cast, the rest doesn't, but still very high quality narration.

Honourable mentions:

• Alastair Reynolds and John Lee collaborations. Lee has the perfect voice and tone for Reynolds' precise (and somewhat aloof) language.
• Neil Gaiman books. Gaiman does his own narration, and I can't imagine his books without his voice.

Dishonourable mention:

• John Scalzi, read by Will Wright. Wright reads as if every line in Scalzi's works was hillarious. Scalzi is a very funny writer, but his books have somber set pieces, and Wright ruins the immersion there.

The OG Wheel of Time audiobooks narrated by Michael Kramer and Kate Reading.

Probably Mythos, Heroes, and Troy written and read by Stephen Fry. He writes Greek Mythology in a digestible, entertaining, but still information dense way, and he's an excellent narrator, of course.

Anansi Boys narrated by Lenny Henry

Normally I like it when Neil Gaiman narrates his books, but Lenny Henry does a phenomenal job with the Anansi Boys characters.

There are so many I really enjoy, these are all great stories with superb narration.

The Dresden Files by Jim Butcher are awesome Narrated by James Masters

The Mitch Rapp Series By Vince Flynn Narrated by George Guidall

The Walt Longmire Series By Craig Johnson also narrated by George Guidall

The Victor the Assassin Series By Tom Wood The Rob Shapiro ones are narrated the best there is another narrator depending on region

The Riyria Series By Michael J Sullivan Narrated by Tim Gerrard Reynolds

The Night lord Series By Garon Whited Narrated by Sean Runnette

The Witcher Series By Andrzej Sapkowski narrated by Peter Kenny

Bobbiverse as previously mentioned

Dungeon Crawler Carl by Matt Dinimon narrated by Jeff Hayes

I have listened to some of these series 3 or 4 times and would highly recommend them all.

Also

Skunk works by Ben R Rich Narrated by Pete Larkin

And project hail Mary as previously mentioned for stand alone titles.

Any of the Lord of the Rings books read by Andy Serkis

I personally cant recommend Siddhartha enough, i found it profound and surprisingly humorous.

The Scholomance series. I can’t count how many times I’ve listened to those books.

Dimension of Miracles by Robert Sheckley

Curse of Chalion by Lois McMaster Bujold

The Girl from Earth's End by Tara Dairman

The Helldivers series by Nicholas Sansbury Smith, as read by R.C. Bray. It's the perfect voice to tell the story of the protagonist.

Anything by Karen Rose. Since we're talking audiobooks, I'd say the Cincinnati series, read by Hillary Huber, and specifically Into the Dark, because the female lead is my favorite of her characters. (You usually want to read from the start of a city, though.) These are suspense novels set in the real world, with pretty dark villains doing pretty evil shit, with characters that are damaged, but work through it in the romance part of the arc.

Another series that specifically is great in audiobook format are any of Janet Evanovich's Stephanie Plum series read by Lorelei King. They're all complete and utter nonsense with wild characters, but the writing and King's reading nail the tone of just taking it all in stride (mostly), and the result is a fun read.

For one more, Dick Hill reading Jack Reacher. These always feel like a light read, which is kind of weird with how brutal Reacher is, how dark some of the organizations he opposes are, and the fact that some of the conspiracies are deep and involved. But Reacher's methodology of steamrolling through any and all takers while protecting the innocent and systematically taking down bad shit that he comes across just feels inevitable. Scott Brick is fine for the later books, and some of the modern crimes are really well portrayed, but Dick Hill is special. I've recently borrowed older books that they re-recorded with someone else, and it just killed the experience for me, partly because it's just worse, partly because I know what greatness sounded like.

I am partial to “Moon Is A Harsh Mistress” because the performer reading all the different parts does such a fantastic job.

The cradle series written by Will Wight, narrated by Travis Baldree. Eastern inspired progression fantasy with fantastic characters and world

The Stand by Steven King And Illuminati by Dan Brown

Were two of my favorites. Bonkers conspiracy stuff nowadays but I enjoyed it as fiction. xD

The Master and Margarita by Bulgakov Was another amazing one.

The Only Good Indians by Stephen Graham Jones. Best non-full cast narration I've heard yet and the book is a rare example of one which is truly better as an audio experience because of accents/local dialect which don't translate as well in writing.