well, I really believe its impossible to pick the "best". the first ones that came to my mind were Liszt, Chopin, Rachmaninoff, Shostakovich, Prokofiev and Ravel.
the first 3 mostly because I'm a pianist and I don't know why I thought of hte others..I just like their music. But who's to say any one is any better than the other? Then there's the usual big 3 everyone knows...Mozart, JS Bach and Beethoven....when all is said and done..those 3 people have influenced western music more than anyone else ever has. So if you want to be "official" you would have to choose one of those 3 guys.