#3 The gravest challenge (1991)
After having made his WWF debut at the previous year's Survivor Series, the Undertaker was announced as Hulk Hogan's title challenger at the 1991 version of the event.
Undertaker would pull off the upset by defeating Hogan and winning the championship, though the match was not without controversy. Ric Flair, who had only recently signed with the company, interfered in the match and cost Hogan the title.
A rematch was scheduled between Hogan and the Undertaker at the upcoming "This Tuesday in Texas" PPV. Hogan had seemingly won the title back but there was so much interference in the match that then-company President, Jack Tunney, declared the title vacant.
Ric Flair was the next to win the championship by winning the 1992 Royal Rumble.