Cena, obviously. Its important when "corporate" storylines like the one with Orton and Daniel Bryan start to not get wrapped up in all the hype. Before I handle Cena, lets take a look at all the other people on your list.
Daniel Bryan: They could quite possibly be trying to pass the torch on to him, and its been said that Cena wants to put him over, but at 32, his window for being the face of the WWE is quickly closing. If its going to happen it has to happen now, and Cena didnt exactly pass the torch at Summerslam.
CM Punk: Punk did great things for the WWE during the summer of Punk, but realistically, his relationship with the WWE has not always been the best. He's taking the #2 spot that Orton inhabited for much of his career opposite John Cena, but thats about it. I would love to see it happen, but Punk would have to move heaven and earth, and his hell of an attitude to get into good graces with Vince. Punk is a product of a system that Vince hates, the self made superstar, much like Daniel Bryan, and while Vince can macho posture around him and claim success for what he's done, Punk would never allow that, and Vince knows it.
Ryback: He's only at the tip of the iceberg as far as what he has to accomplish. The angle opposite Cena was great, and he seems to have a new angle in the works, but he has a lot more to prove before he becomes the face of the WWE.
Randy Orton: That window closed for Randy a long time ago. He seemed like a promising successor to Cena but with the amount of time Cena has remained at the top and Orton has spent with a bland babyface gimmick, it wont be happening. He's closing out his career now as a vet, and will probably hold an Undertaker like role, big backstage, established veteran who can still put over younger talent in the ring, but his chance to grab the brass ring is gone. Sometimes people have to reciprocate, and thats what a lot of fans dont understand. Guys like Orton, Henry, Show, and Kane arent around because they're still optimistic of some day when the company revolves around them. No, the business has made them rich and famous, and its time for them to give back by putting over talent. A lot of guys take their ball and go home (Austin, Rock, Hogan) because they dont feel the need to put others over or they feel threatened by fresh blood. Orton is in the position where he can do wonders for other people's careers, like Bryan's, and will probably spend the rest of his days with the WWE doing just that.
Triple H: Corporately speaking, yes, but as the heir apparent of the WWE empire, I think his days in the ring are coming to an end. Was Vince McMahon the face of the WWF during the attitude era? Triple H can play the crooked boss as well as his father in law, but it doesnt make him the company's star attraction. He knows better than to put himself in the way of the fresh talent. He doesnt always scout the best talent (Sin Cara, Sheamus) which might result in him increasing his on screen presence to do crisis control like he's done now that Cena's gone, but thats about it.
Alberto Del Rio: He's in his prime and doesnt get much of a reaction from the crowd. If in fact he does stay with the WWE, he'll be inhabiting an Orton like role of putting others over.
Rock & Lesnar: Part timers, they're past their prime and dont stand for the company. They have their own individual careers and legacies that transcendence the business. They were the faces at one point, and now they're not.
Ziggler: A glorified midcarder. Every generation has one. During the Ruthless Agression Era you could say it was Shelton Benjamin or Chris Benoit. Forcing these people into the world title picture robs the midcard of men who give midcard titles some good prestige.
Now onto Cena: Hes at the top of the merch sales, the most renown active wrestler in pop culture, and has been at the top of the industry for longer than anyone else. If that doesnt make you the face of the WWE I dont know what does.