There were many books so Giles could find everything he needed. It was in the school and everybody could go there easily.
Then I like the Magic Box.
Then Buffy's house.
I think I don't like season 4 and that's why I hate Gile's house.

It's classic. I like the Magic Box too.

I like the school library as well, but something about this place makes me grin :) It's just so clean and awesome and bright.

I was torn between old skool library the home comforts of hot and cold running popcorn available at Buffy's house. And the popcorn and complex sleeping arrangements of Buffy's won out in my head.
But the library is classic and like seihannon9396 said think of the books, but the same could be said book-wise about Giles' house and the magic box, plus training space at the magic box. Oh clearly I love them all!

My favourite tv hangout ever!

But the magic box was a close second.

I love the library but very so often the mouth of hell opens and some sort of beasty comes up through the floor :P