Dept of Ed data say Iowa was actually best in 2010-2011, data from 2011-2012 probably isn't available yet. http://www.huffingtonpost.com/2012/11/26/high-school-graduation-ra_n_2194378.html#slide=1805697
Maryland, Massachusetts, and New York often do well on graduation rates.
Graduation rate is also a flawed metric in that each state sets their own graduation requirements. I can assure you that it is far easier to graduate in Mississippi or Louisiana than in New York or Massachusetts or Maryland or California.