Although Mjolnir has never been seen in the books, you've all probably seen a couple Marvel movies or Wikipedia has stated that she can level mountains so essentially she is one of the most powerful weapons in all of Norse mythology and is owned by the God of Thunder, Thor.
Sumarbrander/Jack is a sword that was originally Frey's sword and has now been passed down to Magnus. Jack does a bunch of fancy moves on his own and even talks! but he eventually exhausts Magnus if the enemy is much stronger than him.
You can vote for your favorite here.
Don't forget to submit ideas here that are from PJO, HoO, KC, AND Magnus Chase!