I've been addicted to the OST of Metropolis for some time now. Toshiyuki Honda opens with this really up beat dixieland theme with a catchy melody that just seems to make you feel good. As the film goes on, you can hear that same melody redone a number of ways: Light Strings and Piano in "Sympathy", light dixie and jazz in "After all", and the amazing love theme "There'll Never be Goodbye". The score encompasses a wide range of styles ranging from typical motion picture soundtracks to that ever-present jazz influence of Honda. Overall it's an amazing soundtrack and is one of my most favorite.
Also kinda goes without saying, but the music of Cowboy Bebop is also phenomenal. Yoko Kanno has a way of writing unforgettable melodies in practically any style and the ending themes are nothing short of moving. I remember the first times I saw the ending to "Jupiter Jazz Part 2" and "Real Folk Blues Part 2" and the songs just blew my mind.