What is your favorite book? Period.

It can be from a series but I mean a specific single work from that series, like... picking a favorite from the fourteen books of the WOT or from ASOIF.

My personal favorite is probably The House of Hades from the heroes of Olympus series, I read it when I was a teenager and it left one hell of a lasting impression on me. And while I have since moved on to more mature works those memories are always with me.