1 - AJ Styles
As you can see in the video above, AJ Styles and CM Punk have wrestled each other before in Ring of Honor, so a match between the two wouldn't exactly be anything new. However, the two have obviously gotten a lot better since their match in ROH, and of the potential opponents for Punk upon his return, I think AJ Styles could give him the best match.
A unique way to put these two against each other, in my opinion, would be to book them in an Iron Man match. Sure, these matches typically don't do well in WWE just because the audience won't pay attention until the last 10 minutes or so, but Punk and AJ are both so great that they could probably hold the crowd's attention for the full 60 minutes.
Another idea is a submission match, where it could be the Calf Crusher vs. the Anaconda Vice.