Overall records don't tell entire story
These "Big Six" conferences make up 46 percent of the tourney entrants, yet they do 72 percent of the winning. Overall, 70 of the 80 Final Four teams have come from the Big Six, as have all but two of the 20 champions (the only exceptions were Louisville in '86 and UNLV in '90).
Most college basketball pundits would agree that the ACC has performed a notch above the other Big Six conferences in the tourney. There's certainly no lack of evidence to support this conclusion. As the chart above indicates, the ACC holds a commanding lead over the other conferences in terms of its overall tournament record.
But while overall records offer some guidance in determining the top performing conference, they aren't the final word. The problem with overall records is that they don't factor in relative seeding positions. ACC schools that get into the tourney tend to be seeded higher than teams from the other Big Six conferences.