Which fictional universe is the most ambitious and has the deepest lore that you are aware of?
Tolkien’s Middle Earth is the obvious one.
There is Asimov’s Foundation universe which was one of the earliest interstellar spanning sci-fi universes and is still fucking massive by today’s standards.
Dune is also quite large and ambitious.
For more recent stuff, there is Malazan which is the only fantasy series I’m aware of that gives Tolkien’s Middle Earth a run for it’s money in terms of World building. It’s probably the most ambitious fantasy series of the last 30 years.
The Elder Scrolls has the deepest video game world I’ve ever seen, you can spend countless hours reading about ES lore both in game and on the internet.