Thor has a lot of enemies, but none are worse than his brother Loki.
Loki is the son of Laufey, king of the frost giants. When Odin killed Laufey, he took in the infant Loki out of pity and raised him as a son. However, Loki always felt that he was second best to his brother Thor and grew up twisted. Even from a young age, Loki tried to find a way to destroy his sibling and take the throne of Asgard for himself.
Loki has been a thorn in the God of Thunder's side since his very earliest appearances. The power of Mjolnir means he rarely ever tries to take Thor head-on though. Instead, Loki prefers to use his magic and his natural cunning to get one over on his brother and this has lead to some pretty devious plans over the years.
Loki has happily lied, stolen and killed to further his goals and the results speak for themselves. From sending others to do his dirty work to using his own magic to put the God of Thunder through the wringer, Loki has been a pretty bad guy to his brother over the years...