What he has done in terms of accomplishments and longevity its 10, he's won multiple World titles, Main Evented WM a couple of times, he won the Rumble twice and he's done everything you would want from the face of a brand/company to do with the shaking hands and kissing babies (even tho he dont want none).
BUT
Mic work: Has the ability and should be a 10 considering he's been around long enough, but its more of an 7/8. Doesn't really have an classic promos outside of rapping which is over a decade ago, probably his most memorable promo is when he did the heel turn movement. And that he always has to get the last word in his promo face offs, is petty. He is good at manipulating the crowd, but when the crowd is predictable and has been for so long towards you, you should know how troll them sooner or later.
Personality/gimmicks: Gimmick is 5. Hasn't evolved and has been playing the same Cena since he won his first world title, all those years ago, only thing has changed is his outfits, but no character development.
Cena's personality screams heel, he has the smug a$$hole shtick down to a tee, its shame really that he hasnt been allowed to be his true self. (could argue he has been, hes just been presented in a different way then we've been use to.
Ring work 6. More than competent in the ring, cannot lead for shyt, but can have a good fun match if you want to ignore the psychology aspect with his Superman and non selling BS. Put him with a strong to great worker, you can get a great match out of him, but in recent times we've been getting PWG Cena when going up against *insert* made name on indies workers. Doesn't really have an all time great feud and outside of The Rock, his most high profile feud is VS Punk and even then that was more because of Punk and the pipebomb then it being because of Cena. I will give him props for the RVD ECW ONS2 match, just the whole atmosphere would of put a lot of people off, but he did thrive in that situation and didnt shyt the bed and for putting over the the 3 Indy Gods, but even then I think that was more self serving then anything else. But the one thing that keeps coming back to me with Cena's matches and go back to his matches with Brock at Extreme rules 2012, ending of the Nexus Summerslam match and the Wade Barrett match, those things are just inexcusable.
Overall rating 7. He should of done a lot more than he has done and this is a guy that has pretty much done everything apart from a lengthy/strong run as a heel and as the champion being chased. But his career while pushed and over exposed as The Guy for over a decade, doesnt have the classic promos, matches, moments as the legends and icons have. And just like his promos, his feuds/matches he has to get the last word in them also. Aside from Wade, he should really have let Umaga & Rusev go over him, which would of made them look stronger and even more legit as monsters and in the long run it would of made Cena look better when the feud came around again.