And a lot of them start with "A"; Azeroth (WoW), Athas (Dark Sun), Ansalon (Dragonlance), and Abeir-Toril (Forgotten Realms). But I'm going to go with Amber.
I found Amber in my high school years - a series of slender books,written by Roger Zelazny, first serialized in Galaxy magazine. (and which got me reading Galaxy in the first place). It was a great high concept - a multiverse of similar shadow-worlds, differing by details from world to world, and at its center Amber itself, supposedly real and unchanging, ruled by a family of quasi-immortals. Familial conspiracy and swordplay (gunpowder doesn't work in Amber) and tarot cards. What is not to love? The name itself captured both the immortal nature of the setting and how it serves as a trap for those within.
Years later, I discovered that Zelazny was working with the philosophy of Plato's Cave, the idea that reality is just a shadow of a greater, deeper reality. Taking old stuff and reforming it in fiction - cool idea.
|Zelazny's Map of Amber, from Galaxy Magazine|
The original five books of the Amber Chronicles are extremely good and endure re-readings and the passage of time. A second five-book set doesn't hold up as well for me, though I'm not sure if it is because it is revisiting a magical place (making it less magical) or simply that I am not longer the high-school kid who, looking for more fantasty after Middle Earth, found Amber.