Ouch, this is a toughie. Considering what the definition of a professional wrestler is, those being a certain amount of in-ring skills, charisma and mic skills this one goes hands down to John Cena, no question. Let me break it down for you here.
John Cena. I don't like the guy much for several reasons I don't feel much like discussing, but there's nothing you can say against his accomplishments and standing in WWE. He gives everything and gets a hell of a lot back. Sure, he doesn't know eight variations of a suplex but his matches are good enough technically. That combined with the intensity he does bring to the matches creates memorable matches. Simply dishing out a bunch of impressive moves in a match without bringing intensity to it, sort of like Benjamin did when he first started out after being moved to ECW creates boring matches it's hard to get into, which brings me to item #2...
Bryan Danielson. Indie-fans' darling. Among the top of the food chain of workrates in the world. But he has an average look, no charisma and no mic skills to speak of. Danielson is a great wrestler, but there's no chance in hell he could become the same kind of draw Cena is. He doesn't have the other attributes needed to become one of the best professional wrestlers in the world.
In short, under the definition professional wrestler, Cena is the better hands down. Bryan has 100 times his moveset but lacks charisma and mic skills, more often than not irreplaceable traits needed for one to become one of the best professional wrestlers in the world. Danielson is one of those guys who could be great in "real" wrestling but as far as sports entertainment goes, Danielson lacks the "entertainment" part.