As far compared to the other teams that season, it is 2010 Kentucky. We were far and away the best team in the country that season. If we had played best 4 of 7 against WVU, we would have won 4-1.
The 2015 team was better than the 2010 team, but Wisconsin and Duke were really good that year too. So I'm not sure that we would have beaten Duke had we gotten past Wisconsin. Notre Dame damn near shot us down in the 8.
The overall best team in my memory that didn't win though was 1975 Indiana. They were better in 1975 than in 1976, when they did go undefeated. Our upset of them is the best game I've ever seen a Kentucky team play. Keep in mind though that Scott May, college Player of the Year, was trying to play with a broken arm in that one.