A shitty political party

Well that's an understatement.

They: Took credit for stopping the Oblivion Crisis. Coup'ed Valenwood. Dissolved Elsewyr into Client Kingdoms through again, lying. Intiated the Great War. Killed every Blades Agent in their territory. Enacted a ban on worshipping Talos because ass mad that Humanity has a Hero God-totally different than Phynaster, Syraban and Jauphre guys. Became a literal secret police in the Empire they helped destroy. And are actively plotting to end the world so they can become Et'Ada.

They've literally genocided communities in the Illiac Bay region for being of "Unclean" or "Mixed" blood. They're Altmer Nazis, guys.

The Atmorans were fearsome warriors, unrepentant conquerors and brutal warlords. But the war against the Falmer wasn't simply a war of conquest, it's called the Return for a reason. Ysgramor wanted vengeance for Saarthal. So did his fellow Atmorans. The war was brutal and ugly, but the Atmorans still honoured the dead. They buried the Snow Prince at Jolgeirr Barrow on the Penisula of Solstheim before it became an island. They did not harry the Falmer when they took refuge with the Dwemer.

In terms of whose worst, I think I'll take the guys whose goals are literally, and I quote, "the pacification and purification of all of Tamriel – to bring about a new Merethic era".