I mostly listen to Film/Game Scores, Classical, and Jazz. These guys provide me inspiration on a near daily basis:
Nobuo Uematsu- His music is what inspired me to start playing piano as a kid and to begin composing. Huge variety of styles with memorable melodies from some of the most beloved RPGs out there (Final Fantasy, some of Chrono Trigger, etc).
Joe Hisaishi- He's a boss. Composer, conductor, orchestrator, pianist, and he's composed the soundtracks for several of Japan's best selling movies of all time (Princess Mononoke, Spirited Away, etc). His fusion of eastern instrumentation with western harmony is very cool.
John Williams- Sweeping orchestrations, unforgettable melodies, he's a given when it comes to film scoring.
Rachmaninoff- As far as raw emotional intensity, I can't think of anyone more impressive than this guy. He uses some very inventive harmonic material, too.
Chopin- Master of composing melodies. Beautiful music.
Bach- Essentially the father of western music (NOT country western...). Incredible harmonic and contrapuntal inventiveness, and he was a master of keyboard improvisation.
Oscar Peterson- An icon of Jazz piano. One of the great masters.