Sub Mariner is best when he’s teamed up with someone else, so in this story Marvel tries to get some of that Namor fame to rub off on their newest character, Captain Marvel. Namor and Attuma fight for the freedom of the Atlantean people. Then Mar-Vell runs into him and they fight. Not technically a cross-over, but the stories do seem to pair together well.
Creators: Roy Thomas and John Buscema (Subby), Thomas and Colan (Marvel)
For the complete history of the MU, year by year, go here.