Whitney was definitely the best ever, but runner up was Marvin Gaye just because people may have actually gotten busy to that version.
The best I've heard is Dawn Merical - even better when Mark makes it a duet. I'm lucky enough to hear them dozens of times a year and it's goosebumps every time - even if I'm in the middle of doing stuff so we can get started on time and can't stop to actively listen it gets to me.