#4 Randy Orton
Whether you love him or hate him, there is no denying that Randy Orton is one of the greatest superstars in the WWE. He may not feature in the min event scene anymore and seems to have found a home on SmackDown Live instead of Monday Night Raw, but this takes nothing away from the reputation and appeal which the man has.
Reigns has not faced anyone like Orton in his entire career and it would be very interesting to see exactly how he would respond to an opponent such as Orton. Orton does not just bring brutal physicality to a feud - he brings a very strong ( and somewhat twisted) mental game as well.
This is an area which we have not seen reigns thrive in and his feud with Bray Wyatt didn't do much to position Reigns as someone who could play mind games with his opponent and win. It would be interesting to see how he would cope with this in a feud against Orton.