#1 Samoa Joe
Samoa Joe should honestly be a no-brainer for the role of Seth Rollins' challenger. Samoe Joe debuted in WWE in 2017, attacking Seth Rollins. He had a major feud with Shield towards the end of 2017, after which he got injured.
Samoa Joe is too talented to be kept in the midcard, and he is probably the best option to face off against Seth Rollins for the Universal Championship.
Joe is the best superstar in WWE when it comes to mic skills, and his verbal destruction makes for excellent television every time he comes on the screen. He spent his summer of 2017 fighting for the Universal Championship against Brock Lesnar, summer of 2018 against WWE Champion AJ Styles, and should compete against Seth Rollins this summer.
A Samoa Joe-Seth Rollins feud is big money and WWE must realize it sooner rather later!