I was in the theatre when all of these were announced. The audience reaction was palpable in all three cases.
1. The aforementioned Michael Moriarty over Jason Robards; 1973; possibly the biggest upset I remember.
2. George Rose for My Fair Lady (playing Doolittle, definitely a featured role but billed over the title) over either Jerry Orbach in Chicago or Mako in Pacific Overtures; 1976.
3. The two South African actors winning for joint performances in two plays over a tough field including Henry Fonda and Jim Dale; 1975.
And a bonus; In a 1979 tie, Carole Shelley in The Elephant Man (taking a featured role and turning it into a leading one by sheer force of talent) and Constance Cummings in Wings (a tour de force). Who says there is no God?
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher