Whitney has the best vocals live. One Moment in Time and Didn't We Have it All pierce through my poor soul
Adele has soul. Her songs have depth.. They leave you sick or guilty. However, she's one dimensional. Never heard her do something like Migrate, It's Like That, Billion Dollar Bill or I Wanna Dance With Somebody. Adele is lucky to cater to every genre, age and culture.. It's very rare.
Mariah's is versatile.. She can kill any genre.. She knows what to do with her voice.. Her low register on 'The Roof' and 'Bliss'..

She has mastered her vocals pretty well and has the best music out of the 3..