Virginia has a winning record against just 2 schools, but are .500 against 2 others. One is newcomer Syracuse, against whom they are 2-2. The other is against oldliner Duke, with whom they have split 66 contests.
BC and Syracuse both have winning records against 3 current ACC members. But Syracuse is on the verge of 2 more. One with the aforementioned Cavaliers, the other with the Carolina Tarheels, with whom they also have split 4 games.
The 'Heels could add one more winning record if they handle the Orange the next time they meet, breaking that 2-2 stalemate. That would add one more winning record to the 6 they currently have against conference foes.
In just 4 total games against Wake Forest, Duke, and Notre Dame, Louisville is 4-0. In addition, the Cards have winning records against 4 other conference members, as well as a .500 record in their "long" (relatively speaking) history with Pitt (8-8).
So the answer: Louisville, with a winning record against 7 ACC teams.