Villanova was an 8 seed when they pulled the upset of the century against Georgetown in '85
Side notes: Kansas was a 6 seed winning in '88, but actually had a marginally worse record than the '85 Nova team (27-11 to Nova's 25-10)
NCSU was technically a 6 seed winning in '83 but I don't believe there were official seedings at the time. They did have a slightly better record than the above two teams though at 26-10
Beyond that the only 4 seed to win was Arizona '97, while 3 seeds were Syracuse '03 and Michigan '89. Otherwise it's been a 1 or 2
Fun trivia: Since the last undefeated team (Indiana '76), obviously there hasn't been another undefeated team, but also there hasn't been a championship team with only one loss. That's where you note teams like Duke '99, Stanford and St John's two years ago or Illinois last year. That's not really an issue this year though.