I would probably have to say The Pillars of the Earth by Ken Follett. I’m a sucker for fat historical fiction books and this one delivered everything I want from that type of book.
I had never read a historical fiction before it. It is so well crafted, showing different characters and their perspectives, all in the backdrop of medieval England.
It's certainly rich, and the trilogy is satisfying to read, but I had to stop with his giant epics because he's just so relentlessly cruel to his characters. Every remotely sympathetic female character is going to spend at least a decade being repeatedly raped. Every ordinary man is going to have his family murdered and be enslaved. Yes the stakes might be elevated that way and for sure his chosen time period wasn't exactly kindly to those at the bottom of the social pile, but after a while it just felt gratuitous and more reflective of some pathology in Ken Follet's mind than any desire to tell a good story.
This is an amazing book and I've read it 10+ times despite it being ~1000 pages. I tried a lot of the "sequels" but they just didn't deliver the same way and felt too samey and not a great continuation.
I loved this book. I read it as a kid and I didn't understand all of it but it was the first book I encountered of that scale and scope and it showed me what books could be like.