#1 Seth Rollins

Seth Rollins is the most popular wrestler on Monday Night Raw, and has been what kept the show going after Roman Reigns' departure.
He's also the most accomplished wrestler on the roster, and has won nearly every accolade in the WWE, except the Universal Championship.
Brock Lesnar is the current title holder, and his history with Rollins makes their confrontation a storyline that writes itself.
When Rollins wasn't wrestling Dean Ambrose in 2014 to 2015, he was a pain in Lesnar's side while The Beast Incarnate was the champion. Rollins continually tried and failed to cash-in his Money in the Bank contract, until his historic cash-in at WrestleMania.
Rollins and Lesnar had a match later that year, but interference from The Undertaker never gave us a definitive result, and many assumed that neither man was giving their best performance.
Since 2015, both men have added more accomplishments to their resume, including Lesnar's stranglehold on the Universal Championship, and Rollins' dominance in the the mid-card and tag team divisions.
Rollins came off as a coward while he was running away and hitting Lesnar with sneak attacks back then, but Rollins' character has abandoned those traits and has continually called out Lesnar for not being a fighting champion.
WWE need to make Rollins' status as the top star more definitive, and nothing would prove that point better than winning the 2019 Royal Rumble, and winning the Universal Championship at WrestleMania 35.