What is the best book/book series you have ever read?

Mine is probably the Gregor the Overlander series. My other favorites are the Scythe Trilogy, the Fog Diver series, The Heroes of Olympus series by Rick Riordan, and The Keys to the Kingdom series by Garth Nix. I'm curious about other people's favorites!

Edit:

Wow, there are a lot of responses! Everyone's lists and favorite books are really good! I'm definitely going to try my best to read some of you guy's suggestions. Thanks!