As soon as I looked at the choices, I immediately went for The Magic Box. But when I started to think about them, I found myself actually rather attached to Giles' and Buffy's houses. I loved the idea that the Slaying was slowly encroaching more and more upon their lives. It progressed from being a monster of the week thing that they could handle between classes to, basically, Buffy's only function. I thought the invasion of their homes (first Giles', as he wanted to keep it away from her, but inevitably her own) was a very good way to show this.
But The Magic Box was just so much damn fun. Season 5 is my favourite season though, so it's no wonder I like it so much. It allowed for serious discussion, nice training scenes that weren't kind of ridiculous (like, say, kickboxing IN A HIGH SCHOOL LIBRARY IN THE MIDDLE OF THE DAY), and a whole bunch of silly magical gags. Like Anya and Giles summoning rabbits. Or the mummy hand. Or even just Willow being completely unable to gift-wrap.
I don't have much attachment to the Library, I'm afraid. I didn't really mind seeing it blown up.