#1. Seth Rollins - 2019

WWE continued their efforts to coronate Roman Reigns, but after his unfortunate leukemia announcement, the company had no idea when their biggest full-time star would return. Given the seriousness of the issue, they would have had to operate under the assumption that he wouldn't be returning for a long time.
Seth Rollins was the natural pick to take his spot and was always the second-most pushed full-time Superstar behind Roman Reigns. His momentum would lead him to a Royal Rumble win in 2019 before challenging and dethroning Brock Lesnar at WrestleMania 35.
Brock Lesnar changed major Royal Rumble plans? More details HERE.
He would lose the title back to Lesnar a few months later via a Money in the Bank cash-in, which led to a rematch at SummerSlam, where Rollins became one of the few Superstars to pin Brock Lesnar clean. Despite this, his popularity would dwindle a month later after making unpopular comments on social media.
Many fans also felt that The Architect was stale as a babyface Champion, leading to him dropping the Universal title to 'The Fiend' Bray Wyatt a month later and turning heel before the end of the decade.