The problem is, no one really knows. Sure, we can all look at some really messed up things, like the Oklahoma City bombings or the Dahmer killings, and say those are truly heinous, but really, it's all a judgment call. Quite literally in fact -- most states and jurisdictions have special provisions for the harsher punishment of crimes that are truly heinous.
Problem is, like so many things, when you leave this up to an individual person to decide, you get quite different results. And not too surprisingly, the crimes judged most heinous are often the crimes committed by the most socially maligned groups of people.
Well, in an effort to correct some of these imbalances, a noted group of forensic scientists are working on what they call the "Depravity Scale." This scale is attempting to create an objective measure of what the most heinous crimes truly are as a way to make sentencing more fair and focus on the heinous aspects of the crime in question rather than the individual in question.
And you can do your part by taking part in their survey to get a general measure of what American society finds truly heinous. Once enough data is collected, they should have a good view of what the American body politic finds most objectionable, and therefore a much more objective measure of what should induce those extra punishments.
If ever you've thought something was your patriotic duty to help correct some of the most glaring imbalances in our nation's criminal justice system, this would be a great place to start...