A tie between Salem's Lot and 30 Days of Night.
I read the Vampire Chronicles (all of them) and thought of them as an interesting debate on the concept of immortality. Not really a vampire book.
The Charlaine Harris books are good, but they are not exclusively vampire books.
There is always the required reading of Bram Stoker's Dracula. Is it the best? Debatable.
I enjoyed King's Salem's Lot because it held good suspense and the edition I had broke the chapters up in "time of day". That being the case, you wondered who was safely inside their home when the vampires came out.
30 Days of Night was particularly interesting as the concept of Alaska being a breeding ground for vampires 24/7 and the population having to survive until the sun came up a month later. The graphic novels of this have been particularly good.
One book that will NEVER EVER BE ON MY LIST are the Twilight books. My wife and I bought the set and in our extensive library of books, they are the only books we've thrown out and will not keep in our house ever again.
Sorry, I get a twitch every time I think of them.