#1 Triple H
If WWE wants to do a reality-based feud if CM Punk comes back, then there's no more perfect opponent for the ex-WWE World Heavyweight Champion than Triple H, who Punk claims he had major heat with during his years in the company.
Of the possible opponents for CM Punk, Triple H would probably be the only one who can hang with him on the microphone. Of course, if the two did want to bring their real-life heat to their on-screen feud, then their exchanges on the microphone will be extremely heated, which would only bring more interest to the potential match or matches.
It's hard to imagine WWE putting CM Punk against anyone else if he does return to the company, as this would probably be the biggest possible match they could do with Punk, unless "Stone Cold" Steve Austin returns to the ring, which at this point seems highly unlikely.