Sithis is neither an Aedra nor a Daedra. Sithis is the birthed soul incarnate of Padomay and the equal yet opposite force to Anui-El, who is the birthed soul incarnate of Padomay’s opposite force, Anu. It is implied that the Daedra may have been created from Sithis.

Shadowmere or Arvak. I personally like Arvak more, since you can summon him wherever you want, but Shadowmere is stronger in battle.

To get his clothes, Cicero can be pickpocketed, giving him the poison. If the Ghost of Lucien Lachance is active while exploring the Dawnstar Sanctuary, Lucien will tell the player that it is not wise to kill Cicero, because the Keeper is a sacred position in the Dark Brotherhood.

That means Sithis is currently the strongest being he isnt a divine or a daedric prince but he is a deity above them all. Unless if he is the opposite of Akatosh , the leader of the daedric princes or something.

Immortality: Unlike the Daedric Princes and Magna-Ge, the Aedra are immortal in a more conventional sense. But due to him sacrificing his power to create Mundus, Akatosh is still less powerful than the Daedric Princes despite what was shown in Oblivion, even while being strong enough to keep them from invading again.

Talos or Vive is the most powerful I think. Both have CHIM which is the ability to warp reality to your whim, Vivec is using it to mouth bone Deadric Princes with his spear and Talos is using it to literally hold reality togeather.

The weakest are Peryite, Namira, Nocturnal, Malacath, and Meridia. Sheogorath and Vaermina are about equal, since both can cause madness through different methods.

He disappeared for ages and many thought that he lived eternally. This wasn’t, in fact, true and simply hid away until he was ultimately slain by the last Dragonborn. The Dovahkiin are immune to everything a normal human would be immune to. They’re not immortal, they’re not invulnerable.