The Inquisition has to rank pretty high but perhaps even worse than that was the European exploitation of the undeveloped countries during the age of exploration. And of that exploitation, the Spanish plunder of South America probably leads.
The worst injustice is man on man crime. It's been an ongoing travesty, whether it is/was slavery, the Holocaust, etc.
Taking Pepsi Clear off the market!
In modern times probably when all the Katrina refugees tried to walk out across the bridge to get out and were turned back by police at gunpoint, then forced to stay in the dome where assaults were happening regularly.
Slavery and totalitarianism in all it's hidden forms.