All the conquerors and warlords in history have caused millions of common people to suffer. All empires were established on mounts of corpses and cemented with blood. Hence you can count the common peoples of all countries and all nations among those who have suffered historically.
But there are some who have suffered more than others. The native people of both the Americas, the black natives of Africa etc. come to the picture first. The native people of Asia and Oceania who came under colonial rule also suffered to a lesser extent.
The victims of conquerors like Alexander, Jhengis Khan, Timur and many lesser warlords and the victims of Nazi genocide have all suffered. If you count religious groups,the Jews are in the forefront of minorities who suffered.The Zoroastrians have also suffered when Persia came under Islam. The spread of Christianity and Islam has caused the suffering of the followers of ancient religions in many countries. Thus listing of victims in history is very difficult.
Now to answer the second part, it is even more difficult. Hence I leave it for the time being. But I feel sorry that some ancient empires and civilizations have fallen. The Indus Valley Civilization and the ancient Egyptian Civilization are the two ancient civilizations I would like to point out. The Zoroastrian dominated Persian Empire, The Mauryan Empire, The Chola Empire, The Vijayanagar Empire, The Khmer Empire, The Srivijaya and Majapahit Empires, The Inca Empire and The Aztec Empire are some of the ancient empires I would have liked to last longer.