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

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

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.

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

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.