Punk drops it to Rock at Rumble. Later in the night, Punk wins Royal Rumble and challenges for WWE title. The debate lands in which number you place him at.
30, for example, would bury the entire rumble. That would be no good.
#1, would have him come out first and would be highly unbelievable for him to win after having a match with Rock and withlasting 30 other men in the same night. Not to mention how fucking tired he'll be. Yet, it goes with his gimmick that he is able to do these things because he his "the best wreslter in the world."
I'd say about half way through the Rumble at or 14 or 16 would be the best option.
Now before anybody says that oh I'm just a punk mark, punk has never won a rumble and if he should the time should be now when he's red hot. Its a big enough swerve, (winning rumble) but believable enough so.