Delurking to chime in with some of my favorite worlds/authors.
I thought someone would chime in with Tamora Pierce before now. I love the Circle of Magic/Circle Opens/Will of the Empress world, and also the Lioness quartet, Trickster's Choice/Queen, and Protector of the Small quartet. I feel very very dorky going into the teen section of the library (usually with baby in tow) to get them, but dorkiness be darned, I do it anyway.
Gareth Nix-Sabriel, Lirael, and the Abhorsen. I love this world. (Would love to run an game in it!) Haven't been able to get past Monday in the Keys to the Kingdom series, though. I should retry it. Didn't like Ragwitch or the Tower books at all.
HP and Auel, of course. The Anne of Green Gables books--Rilla of Ingleside is my go-to book when I need to cry and cry and cry and wallow in the crying. Darkover, and Outlander, and King pre-Needful Things (quit reading Dark Tower at Wizard and Glass), McKinley (love Spindle's End, up to the end--I end up putting the book down about 20 pages before the end & just never pick it up again), Heinlein...
Oh, and thank you all for naming your faves! I've kept a list I'm running to the library for. Whee! New books!