This is an interesting list:
6. John Cena loses to The Rock at WrestleMania 28
5. Randy Savage loses to Ultimate Warrior at WrestleMania 7
4. Ric Flair loses to Shawn Michaels at WrestleMania 24
3. Steve Austin loses to The Rock at WrestleMania 19
2. Booker T loses to Triple H at WrestleMania 19
1. Undertaker loses to Brock Lesnar at WrestleMania 30
I won’t argue too much except to say that Sting losing to Triple H at WrestleMania 31 should definitely be on it.