Great in ring storyteller, awesome at playing up to the crowd (whether they love him or hate him), has a more versatile moveset than he's given credit for though his character doesn't call for it, not so great at selling imo, and hasn't ever put on what I'd call a 5 star match. I'll go with B-.
Other than long time Triple H and Undertaker, he has the best matches.
Cena vs HHH WM
Cena vs Edge vs HHH BL
Cena vs RVD ONS
Cena vs Edge TLC
Cena vs HBK WM
Cena vs HBK Raw
Cena vs Orton vs Edge vs HBK BL
Cena vs Khali (Khali's best match) JD
Cena vs Lashley (Lashley's best match) Bash
Cena vs Umaga (Umaga's best match) RR
Cena vs Orton SS
Cena vs HHH NoC
Cena vs Tista SS
Cena vs Jericho SS
Cena vs HBK Raw II
Cena vs Edge BL
Cena vs Swagger (Swagger's best match) Raw
Cena vs Orton BP
Cena vs HHH Raw
Cena vs Orton BR
Cena vs Tista WM
Cena vs Tista ER
Cena vs Barrett (Barrett's best match) HitC
All of those, with the exception of Cena/Khali, are four stars or above. That's not including his pre-SuperCena matches such as Cena/Lesnar, Cena/Taker, Cena/Angle, etc.
Although he never had a five star match, or cannot seem to grasp the idea of long-term selling, he is a great wrestler. With the exception of DB, Jericho (when he is here), and a couple other wrestlers, Cena is the best wrestler in the WWE. I definitely have Cena in my best top 5 wrestlers list atm.