In general, it would have to LOTR. Because I've read one copy into the ground.
But this year, my top three Halloween fitting titles would be:
The Women of the Otherworld series. - I believe this was Armstrong's first series. At times it shows, but you can see her developing as a writer (and she works with her editor). What I really love about this series is not only does feature women, but each woman is strong in her own way. And I say this as someone who didn't like all the women in the series. Additionally, the women are friends with each other.
Merrily Watkins - I've developed a love for series, mostly because of the characters and the use of English folklore.
Count St. Germain Series (counting the Olivia books) - world's greatest vampire.