I would put the Mizzou Thetas up there. In Spring 2011 they had a 3.44, which was a drop from Fall 2010 which was a 3.49. The chapter size last fall was 239 with a new member count of 79. In Fall 2009 they had a 3.526 and all sorority was 3.221.
They have been first in grades on that campus for what seems like forever.
link to all grade reports
ETA: It would be hard to compare "top grades" across the country due to potential variances in grade inflation, etc. Perhaps someone could determine who has the highest standard deviation above the mean? I would, but I am having too much fun on polyvore these days.