The Julio-Claudian Emperors
It has been 35 years since I took a Roman history class--and I know that I have forgotten a bit of what the Julio-Claudian emperors did, but I know that I didn't forget this much. While the theory that lead leached from pipes caused their bizarre behavior seems to be pretty well disproved, the explanation that lead leached from the pots in which grape juice was concentrated for fortifying wine poisoned them, and drove them insane, seems to be rising.
Suetonius's accounts of the misdeeds of these creeps are so shocking that you really want to believe that he is mistaken, repeating horrendous gossip. This account from Suetonius of Nero's dressing up as an animal and what he did while so dressed is so weird that I can't imagine anyone inventing it--something that so weirdly warped that today, you would have to go to San Francisco to find. And Suetonius also tells us of history's first same-sex marriage to Sporus--but so gross and barbarous that even San Franciscans might say, "Wait a minute, that's just wrong! (Did I say that?)"
But what will people of the year 4000 say when they read of the Holocaust, or the tortures of Saddam Hussein's government, or a myriad of other horrifying crimes?