#1 Randy Orton
This is John Cena's longest rivalry till date, against The Viper. Both have brought the best out of each other throughout the rivalry. John Cena hasn't faced any wrestler in his career more times than Randy Orton.
This feud is going to be remembered for generations. Seven solo matches, that is still quite a lot of matches for one rivalry to have.
It should also be noted that with the exceptions of the Royal Rumble matches, the WWE title was on the line for each match.
For the triviality of it, the first time that they were involved in the same match on PPV is the 2004 Royal Rumble.
Orton entered that match at number two, but is gone by the time Cena entered at number 28. So it is notable just for that statistical fact, as it wouldn’t be until 2007 that the two would be in the same match on PPV together again.
And that would begin a war that still rages. John Cena's greatest rivalry of all time.