Locrian, the most dissonant mode
Locrian is the rarest of all modes. I do not know of one single piece of music written in Locrian. To get a feel of how hard it is to write in this mode, try to write a tune that starts and ends on B. Use no sharps or flats, no black keys. Use no chords that are not white. Remember, you have to start on B and end on B and convince yourself and others that you have written something pleasing to listen to.