#1 The Undertaker

This one should be a no-brainer. There's no veteran in the locker room, and perhaps no superstar in the history of the WWE locker room who Vince McMahon respects more than The Undertaker.
Not only has he been with the company for over 27 years, but he established himself as a true locker room leader and legend of the business. McMahon often says that The Undertaker is among his greatest creations, and he isn't wrong.
Anyone could have taken that gimmick, but it took special talent from Mark Callaway to own that character and make it into the mythical figure that it is today.
As for The Undertaker's future in WWE, we're really not sure. It seems as though he may have quietly retired, or decided to be careful with the dates he chooses. He worked a lot in 2018 and 2019 may be the first time in 27 years that we don't see The Undertaker at all.