Oh, what a frustrating question this one is. I grew up on Dungeons & Dragons so I’ve read a lot of those early novels when the first Weis & Hickman books arrived and so forth. I spent years reading Myth books from Robert Aspirin and I’ve gone to the classics and everything in-between for the longest time, though I’ll admit I dropped out of fantasy books a good 15 years ago. The last serious series I read was the Eye of the World Robert Jordan one but I never finished with the books that were put together after his death. I did read Game of Thrones but that in a weird way doesn’t feel like it counts for me since it was research for the show for work.
One of my favorite sprawling properties comes from Raymond E. Feist with his Riftwar stuff because it jumped all over the place and delved into a lot of characters and times as opposed to something singular.
But even within that, I have a favorite from what I had read and that’s the Serpentwar Saga that was made up of four volumes between 1994 and 1998. Just everything about it clicked in the traditional young man’s journey into the world and discovering truths and more, but having it set against this larger backdrop of interconnected stories made it incredibly lived-in.
Bonus question: Which fantasy series have you read that left you the most frustrated for squandered potential/storyline?