Whitney.
Whatever the greats could do well, she could do to a great degree. Her voice was rich and powerful. Yet her tone could be sweet when needed. She could run when needed and her vibrato was great. She had a great lower voice and great head voice. When she needed to growl and take it there, she could. Not to mention, she had great range.
All the things we look to the best singers for as far as their strengths, she could do all of those things pretty well. There will probably never be another voice like hers and that's why I don't really think any singer can do her songs justice the way she did....no one. They just don't possess all of the things her voice had.
Her albums may have left much to be desired, but in her prime, her voice was amazing.