Yeah, this game has been played on these forums a few times before... and my answer has always been:
A truth telling machine. One that is absolutely 100% correct. Most of the problems in this society would be solved by such a machine. No more innocent people incorrectly incarcerated. And criminals would never escape proper sentencing because of good lawyers (eg, OJ Simpson). Treaties between nations might actually be kept... and even wars might be diverted if we know whether a nation like Iraq really is trying to build the bomb or not. Politicians might even be a bit more effective, because we'll know whether their actions are based on personal interest or the interest of the country. People might even become a bit more ethical. Your kids will be less likely to do drugs or criminal activity if they know they'll get caught (and so would everybody else).