The Kekkai Sensen OST is one of the best anime soundtracks I've heard. Taisei Iwasaki caught the vibe of a post-apocalyptic New York scene really well. Also, it is home to one of the most iconic anime endings out there, and I'm a little surprised that despite that it's actually pretty obscure. Its OST is inspired a lot by big band jazz and other music genres, I think some of its best tracks would be Catch Me If You Can, WHITE BEYOND, Snap Out, and its main theme.