I wouldn't say that when it comes to Freyja or Freyr that one is superior compared to the other. They are brother and sister, children to Njord. All three became Aesir during the conflict between the Aesir-Vanir. Freyja was often associated with love and beauty (as well as death). She taught the practice of seidhr to Odin. Freyr was often associated with fertility and harvest. You asked later on if Gerd was a goddess? Yes. She's the wife of Freyr, a jotunn whom he wooed. There's a lot more to all three of those deities, but that's the short version.
Sif is the young and beautiful wife of Thor, known for her golden locks. There isn't an incredible amount mentioned about her in the lore. She is sometimes associated with fertility and crops. I've never personally looked much into her, so I would suggest again that you do some research if you're more curious.
Odin's wife is Frigg, whom is a motherly goddess. She has several handmaidens. In many ways, she is kind of like a queen in Asgard whom overlooks the ongoings that occur, gives her husband council, and is well-respected. She was a goddess to mothers and wives and home-related matters, often called upon in childbirth and around the hearth. Much like her husband, she is also said to have a magical inclanation.
Skadi is not Odin's son. Skadi is a female jotun whom later became one of the Aesir. Her story is an interesting one, in my opinion. She is a goddess of the hunt, associated with ice and winter. She originally marched to Asgard against the Aesir after the death of her father (you should read more about this). Through an arrangement, she came to marry Njord but their relationship ended after not too long. She stayed with the Aesir as they valued her counsel.