No, actually, she's not. Mariah only has about 1 half step larger range then Michael. Michael has a range of 4 octaves, about, Mariah has a range of 4.1. And considering Michael's technical brilliance in transitioning between registers, the control his displays, the timing and breathing technique, no one is technically better. Whitney has a range of about 3 octaves. Michael is superior to her as well, in every way.
http://www.youtube.com/watch?v=Q5GSxSmYvME&feature=related
http://www.youtube.com/watch?v=VUoEil40qZA&feature=related
mj's voice doesn't come close