#1 Baron Corbin
It's hard to admit when a heel is at the top of his game because if he/she is doing his job well, then you won't like them. Sometimes it's easy to see when a heel is cool, or a good worker, and fans begin to like them. This causes a blurring of the lines for the character, which isn't really what WWE is shooting for.
They have hit it out of the park with Baron Corbin (now King Corbin). Fans routinely boo him, he's an incredible instigator, and he is one smug king. He was certainly deserving of the crown of 2019 King of the Ring. Corbin is arguably the top heel in the company.
CM Punk used to be a very popular Superstar, now he's a man of legend amongst the WWE Universe. He represented the jaded fan, he allowed the disgruntled viewer to live vicariously through his rebellious romp through WWE. Pitting him against a much-maligned product of the WWE machine would provide even more fan support.
I can only imagine the pipe bombs Punk could drop on the former RAW General Manager...
Don't forget, Corbin is no slouch in the ring. While drawing a chorus of boos and jeers, every one of his King of the Ring matches received (and deservedly so) chants of "this is awesome!"
King Corbin is the best fit to square off with CM Punk in a return to the ring. At this point in his career, he is the best heel WWE has to offer up to a huge babyface. The crowd hates him for all of the right reasons.
All hail King Corbin!