I think it's graduation rate. Bamspeedy did a good job explaining how using post secondary can be tricky (so many variables to track), so if our intent is to track state by state, then we should use only K-12 data.
ACT/standardized test score data can paint a useful picture, but for all intents and purposes, I think ACT score, if we're looking at large populations of people, kinda has a diminishing utility above a certain score...we don't need every student to qualify for Harvard. If a state's public school graduates are competitive applicants for their state universities, then I think that school is doing an excellent job. To give you guys a ballpark figure, a very good public district will have an average ACT score of around a 24-25. I don't think any states, are above that. The National Average is around a 21.
Graduation rate tells a lot about the achievement gap in a state...what's the difference between the "good" schools and the "bad" ones? Are the lowest achievers still able to to meet the min benchmark? I'm sure that there are wealthy suburbs in places like Alabama and Arkansas that have good schools, but when over 30% of their students can't meet the 10th grade benchmarks, I think it's hard to say that their STATE is "good with education". States like California and New York (high wealth states) also are going to have very good schools at the top end...but that bottom end is awfully large.
I think if you look at graduation rate, and to a lesser extent, standardized test scores, you find that schools in the northeast and the northern midwest/plains states tend to be the best for education, and schools in the west (nevada, new mexico) and the south tend to be the worst.