#2 Daniel Bryan
These two have a history with one another that dates back to their time in the indie scene. Both took similar paths to get to the WWE and both men rose to heights in the company rarely accomplished by men their size.
I know Daniel Bryan and CM Punk had numerous matches against one another during their time in the WWE, but I don't care. This is a match that needs to happen if Punk decides to make his comeback. These two can put on a technical wrestling clinic. There's a chemistry between them that is hard to replicate. It would be a disservice to the fans not to put this match on.
So why would Punk return and start something with The Planet's Champion? Well, Punk's departure from the company in 2014 did cause WWE to call a number of audibles when it came to WrestleMania 30. You could even argue that Punk leaving opened the door for Bryan to assume to the top spot in the company. Surely creative could build a story from there.
Frankly, I don't think there needs to be a reason. Maybe Punk wants to come back to face his old friend one more time. To put on a match that no one thought they'd ever see again. That is a good enough reason for me.