#1 Seth Rollins

The man who was going to beat Brock Lesnar wasn't going to be just any superstar. It was already picked to be Reigns throughout 2018, but once he had to relinquish the title, we know how things played out. Rollins 'put Raw on his back' and routinely had matches of the night throughout most of 2018 and 2019.
He was already a made man due to his status in both the Shield and the Authority, but now he truly gets to do it on his own. We already knew he was a great performer but always being a part of a team can sometimes hold someone back.
Rollins was picked to both win the Royal Rumble and dethrone Lesnar. Although the first one doesn't always translate to success (See Shinsuke Nakamura, Asuka), the second one always does. Lesnar won't agree to lose to just anyone, so the fact that Rollins defeated him means his reign will likely be more than just a defense or two.
His status in the company and ability to perform at the highest level means he should hold the title for most of the year, but if a ruthless man from Scotland wins the MITB briefcase, it might be a shorter reign than he should have. Regardless of who wins the MITB match, Rollins is the focal point of Raw and will continue to be for at least the next year.