Loki (and other tricksters)
First things first: separate the Norse god Loki from the Marvel villain Loki in your head. Though one is based on the other, there is no way in Jotunheim that the Avengers could best the Norse original, who was decidedly not a puny god. The proof is in this picture of Loki eating a giantess' heart, which impregnates him with Fenrir (that's right, Loki himself gave birth to the aforementioned world-eating wolf, as well as an enormous serpent for good measure). But Loki's real power — and the power of other trickster gods like the Navajo Coyote, the Haida Raven, or the European Reynard — is manipulation. Superman is super-smart, but he could never win a battle of wits against Loki. The trickster god's insane charm would have Superman hypnotized faster than a speeding bullet, adding one more soldier to Loki's family of chaos monsters.