#6 The 2015 Royal Rumble match
After the breakup of The Shield in 2014, it was obvious that Roman Reigns was the top priority in WWE. With a movie star look, Vince McMahon wanted Roman Reigns to succeed the aging John Cena as the face of WWE. Throughout that year, all of the pieces were coming into place. Cena was squashed by the newly-godlike Brock Lesnar at SummerSlam, having his title taken from him. Roman Reigns was rocketing up the ranks. It was obvious where this was going.
The problem was that Vince McMahon hadn't consulted his company's fans, who, having found a new voice in 2011 and using it to alter the landscape in 2014, had something else in mind. They wanted Daniel Bryan to match up against Brock Lesnar in a David vs. Goliath encounter and win back the title he never lost.
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The Royal Rumble match in 2015 again saw these tectonic forces rub up against each other and create an earthquake, even bigger than the last. After Bryan's elimination, it was obvious who was going to win the Royal Rumble, and the same Roman Reigns who had a good reception the previous year got jeers. Even The Rock got jeered when trying to help him.
Most people believe that 2015 was the worst Royal Rumble match ever, and though Roman Reigns' popularity was waning in the closing days of 2014, this match was the decisive moment that soured fans on him for the long term. He would be fighting from underwater for the next four years, and it would make itself felt.