This is a question I fear cannot be answerd due to how close it is. I would love to go with Taker as in my opinion he has given so much to the industry in the past two decades. Having said that, to denie John Cena's acomplishments in such a short period of time would be absurd. I know this thread is going to be full of Taker marks giveing it the five moves of doom bull, but I am going to take a look at this subject regarding two key points.
Drawing Power
John has to take it as far as drawing power goes. This is not to say The Undertaker is not a huge draw, he is a very very big draw, especially around Wrestlemania time. However in the year 2004 when Lesner decided he could not be assed with wrestling anymore, John Cena put the company on his back around a year later. Back in the ninties Undertaker had Hulk Hogan and Randy Savage, and near the end of the decade had The Rock and a certain Stone Cold Steve Austin. Not bad in all honesty compared to what John Cena has to work with(Batista,Triple H). That is no disrespect to those wrestlers, but compared to the earlier list, they cannot compare. So as far as drawing goes, John takes this catogory becuase not only has he taken the company such a long way without such a supporting cast, but also becuase he brings in a ton load of money throguh merchindice sales and so on.
Reputation
This one I will keep short, becuase we all know the current reputation that Cena holds, especially on the internet. "HE ONLY KNWZ 5 MOVES!" etc. Althroguh that statement is complete and utter bull, it is indeed what the currect fan base(again, speaking from internet perspective) think of John Cena. Where as the Undertaker has a fantastic reputation not just here, but in the whole wwe universe. Undefeated at Wrestlemania, classics with Shawn Michaels in the current two years, and the ability to keep such a rediculous gimmick not only so current, but pretty damh good. The Undertaker has a far better repuation that John Cena, which is due also to the fact that for having been around for two decades, he is respected.
I am sure I have missed things out here, so feel free to add, debate or correct me. In conclusion I feel that this is too close to tell, becuase even throguh we should be looking at it as how it stands just now, the thought of what it will be in ten years is always looming.
This is just too close too call.