#1 Not Roman Reigns
While Roman Reigns is one of the most popular superstars on the entire roster, there is still no doubt that a portion of the audience resents him. Maybe that has to do with how he was pushed by management before his leukemia diagnosis, but WWE doesn't need to make this situation any worse.
With that being said, WWE needs to avoid putting Reigns in the title picture at all costs or risk building up that same feeling of resentment. Seriously though, if WWE were to have Roman Reigns face The Fiend at WrestleMania 36 and be the one to end the streak, he will again go back to being despised.
That's why WWE needs to have The Fiend face The Undertaker and keep the streak going past WrestleMania 36. At least that way, WWE can protect The Fiend's streak heading into the summer months, The Undertaker can retire with dignity and Roman Reigns lives on to fight another day.
In the end, WWE has two choices. Book Roman Reigns to defeat The Fiend at WrestleMania and risk building up that resentment factor. Or go the safe route and allow the torch to be passed in a meaningful way.