2000 PGA Championship at Valhalla. Tiger Woods and Bob May battled head to head on Sunday and set a new record for lowest score to par in a major championship over 72 holes (broken in 2015). Both shot 31 on the final nine.
Tiger won the playoff, becoming the only man in history (still) to win three consecutive professional majors (not counting 19th-century players who won three Opens in a row when it was the only existing major, was contested over only 36 holes, and had as few as 8 golfers in the field, all from England or Scotland). It was his fourth win in five consecutive majors, and he went on to win the next one (2001 Masters) as well, making it four consecutive majors, and five out of six.
Also, Ali-Frazier 1, Foreman-Frazier, and Ali-Foreman.