It's quite possible that Gorr the God Butcher might be the most dangerous and personally damaging villain Thor the God of Thunder has ever faced. Armed with the lethal All-Black the Necrosword, Gorr subscribed to a philosophy of misotheism, meaning he had an intense hatred of any and all gods, after enduring an incredibly tragic personal loss that came after his prayers went unanswered. As such, Gorr sought to seek out and kill every God he could find. While Thor was eventually victorious and did manage to best the God Butcher, Gorr left a long-lasting mark on the Son of Odin, and was even the reason Thor became unworthy to wield his own hammer for a time in Marvel Comics.
Gorr is a creation of Jason Aaron, making his first appearance in Thor: God of Thunder #2. After the horrible deaths of his wife and family, he believed that the gods couldn't possibly exist seeing as how his prayers for them were left unanswered. For this, he was banned by his tribe. However, when he discovered that the gods did exist, but still didn't help his family and others, he became enraged and swore to kill every god he could find. After gaining the Necrosword from Knull the Symbiote God, Gorr gained the means to fulfill his dark quest. However, after multiple clashes with Thor across the God of Thunder's life, Thor did manage to defeat Gorr before he could set off a "Godbomb" that would have killed all of the gods simultaneously.
However, Gorr had one final blow to deliver to Thor from beyond the grave. Shortly after Gorr's defeat, Thor participated in Marvel's Original Sin event, and Nick Fury took advantage of the uncertainty and doubt his interactions with Gorr had caused within the God of Thunder. A small part of Thor had been dwelling on Gorr's belief that the gods weren't worthy of their power, resulting in drastic problem for the Son of Odin with three words uttered by Fury: "Gorr was right."
With those words, Thor's doubts about the gods' and his own worthiness were cemented, and Thor found himself no longer able to wield his mighty hammer Mjolnir. He no longer believed himself worthy to lift it and use its powers, and it was all thanks to the seed of doubt sown by Gorr and his hatred. Though Jane Foster took up Mjolnir for a while, Thor's journey back to worthiness has defined his character ever since, cementing him as the last god left at the very end of the universe in King Thor. Gorr's arguments changed how both Thor and Thor's future writers and readers would understand his role as a divine being: that he was either a god of the people, or no god at all.
With the recent news that Gorr will feature in the MCU's Thor: Love and Thunder played by actor Christian Bale, it will be interesting to see if this particular arc from Jason Aaron will play a part as well. In any case, Gorr is no doubt one of the most dangerous foes Thor has ever faced, but it was nothing to do with the giant black sword he wielded, and everything to do with his dark convictions that rocked Thor to his core.