Its Edge.
#1 Rival, not his biggest rival, his hottest fued, best match or whatever. Its the guy who is or was Cena's main antagonist and its Edge no question.
in kayfabe, Edge and John Cena HATED each other. They'd never team up or never work together, they were direct opposition to each other.
Cena and Orton were rivals, but they didn't hate each other, they commonly team up and worked together in storyline numerous times and even grew a respect for each other. It was more career rivals than outright hatred and Orton's most heated moments were times he rehashed what Edge already did.
Cena and Punk never hated each other, they actually respected each other, Punk just hated what Cena represented. Cena was just one of the cogs in the machine that Punk hated, which is why once Punk actually got the strap, most of his fued was against the authority figures at the time.
Rock was his highest drawing rival, but their fued was based around respect for the business and each other. They also teamed up and eventually went on to become allies in kayfabe.
Edge and Cena NEVER were allies and never respected each other (in kayfabe) and went from validating the entire Money in the Bank concept to actually giving Cena his first legit fued as a main eventer.
The answer is Edge.