#2 AJ Styles

Of all the potential opponents for Mysterio upon his return, he'd probably have the most spectacular match with the current WWE World Heavyweight Champion AJ Styles.
It's pretty much a guarantee that WWE will book a Mysterio vs. Styles feud and/or match at some point after Mysterio's return to the company, barring some type of major injury to one of the two men.
Breaking Vince Russo and Jim Cornette update HERE.
Do I think WWE should have Rey Mysterio challenge Styles for the title? No, simply because Mysterio doesn't need to be world champion. So, if I were WWE, I'd only book this match after AJ loses the title, because the WWE Title being on the line would probably spoil the result of the match ahead of time.
One wrong move ended Big E's career - Watch Here!