Is Cena, hands down, but to say that only he draws is a mistake.
HBK is true, he needs a stong feud to draw, if not he just is someone else in the card.
HHH draws in any match he is, like this Summerslam, he is drawing a lot with his return, I think that behind Cena he is the biggest draw.
Batista, just like his fighting, draws according to his rival, he drawed with Triple H and with Undertaker.
Orton, yeah, he hasn't draw as a champion, but for example in Armageddon 2005(not sure if the buyrates were good or bad) practically he draw the god damn show, well, he and Taker.
Undertaker draws if he is in a title match, is fighting a good opponent or the name of the PPV is Survivor Series.
And you are forgetting about the best heel currently in WWE, Edge. he is a pretty drawable and trustable as a champion and his other feuds are also good. He is practically just starting as a true main eventer but he draws a lot.