It depends, do you mean protected by booking or actual power level? Protected by booking is just whoever is the face champ at the time, like Super Cena or Roman or whatever
Kayfabe "power levels" like Dragon Ball Z? It probably goes something like
1. Mortician Undertaker, he felt 0 pain
2. Ultimate Warrior, unearthly power
3. Goldberg, undefeated
4. Brock, especially when he returned and beat Undertaker
5. Cena, just based on overcoming the odds
6. Hogan, Hulking up and shit
7. Kane, monster but could still be hurt
8. Bret Hart, not in size, but kayfabe could outwrestle anyone
9. Batista, could overpower big men, but not as much as cena
10. Nash, big man who only realistically lost if you caught him on a bad day
Honorable mentions: Stone Cold, Roman, Seth Rollins, The Rock, Eddie, Triple H, etc and HBK; all top guys but were never the highest power levels.
Doesn't take into account really old school territory guys like Road Warriors
Doesn't take into account giants like Andre, Big Show, Khali, Viscera etc
Also I haven't watched too much recently to know where Braun Strowman
