I'd have to say the Canalave Library. First off, the Canalave city music. It's so beautiful. I can just imagine myself leaning over the railing of a boat, hair blowing in the breeze, inhaling the sweet scent of the ocean, and pulling up to the docks...
Second, the books on Sinnoh. I know them all by heart ;) The myths of Sinnoh and it's philosophy always interested me! (Maybe I should start going to church?)
Ultimately, it's a beautiful city, and me loving the ocean, makes it even better.
One of the many reasons I love Sinnoh.