#2 Samoa Joe

Samoa Joe has constantly been a threat to every superstar on both Raw and SmackDown. While he did hold the NXT Title on two occasions, he hasn't been lucky enough to capture a singles' title on Raw or SmackDown.
He debuted on January 30, 2017, when he attacked Seth Rollins in Triple H's name. It took some time, but he eventually challenged Brock Lesnar at SummerSlam in 2017 along with Roman Reigns and Braun Strowman.
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Lesnar retained, marking number one on Joe's list of failed title attempts. Number two would come when he failed in a singles' match for the title against Lesnar.
Joe was shifted to SmackDown Live in the Superstar Shake Up last year and eventually battled AJ Styles three separate times for the WWE Championship. He lost each time.
Since he has been persistently chasing one of the main titles in WWE, it means he's seen as a main-event guy, but since he hasn't captured any singles' titles and doesn't even have a mid-card title run on his resume is a little disconcerting for someone of his talent.