I would like to ask you this. I would like to know opinions about this. What species in Galactic Civilizations suffered the most? Especially from the major civilizations.

My opinion would be Drath. They got almost genocided on their homeworld, Altaria, for no reason other than Altarians feared them. They were evacuated and found a new planet. They planned their return, but Terran stopped them. Despite this, they provided some aid (or at least stayed natural, details here are sketchy) during the Dread Lords Wars. And then, the Drengin defeated them and they were genocided again, this time by the Korath. They managed to find a new planet and start to rebuild, but they were barely up and running by 2245. Despite this, they provided assistance to Jenna Casey’s fleet that year.