#4 Rey Mysterio
SmackDown is amazingly short on babyfaces, and Rey Mysterio recently returned to join the upper mid card on SmackDown Live. Rey Mysterio's last full-time appearance resulted in him being booed mercilessly when he came out at #30 in Royal Rumble 2014.
Every single fan in the arena wanted to see Daniel Bryan enter the Rumble at 30, but got Mysterio in turn. It will be fitting if Mysterio is announced as the next opponent for Bryan, exacting some sort of revenge for the 2014 incident.
Rey Mysterio and Daniel Bryan are possibly the two most loved superstars in the last fifteen years, and are universally appreciated for their tremendous wrestling abilities. Mysterio fighting Bryan will surely generate interest among all generations of WWE fans.
Rey Mysterio has been a World Champion before, and an entry into the main-event picture for him would not be unwarranted at all.