Well, music is the obvious choice for many, because we're so emotionally connected to music, myself included. Literature is, of course, very important.
So I would hold those two above the rest. After that, architecture is truly one of the more interesting things I've ever taken the time to look into. I really do recommend people to learn up on architecture, because it's absolutely fascinating how the simple design of the building can completely motivate a specific purpose and feeling while in the building. I've always loved to look through pictures of Zaha Hadid's designs. Truly revolutionary and progressive.
Music, in particular Orchestral music. It's just something wonderful, hearing a good orchestra. All those different sounds, blending into something beautiful.