I think the way I create my Sims is what makes them memorable to me.
I had a Sim family that I named the Tiger family, and another named the Wolf family. I first created them in my Sims 2, and recreated them in my Sims 4. The Tiger family is made up of a mother that I always thought of as a widow, and her five kids. The Wolf family is made up of parents, and their first four kids. In sims 2, one of the daughters of the Tiger family, and the son of the Wolf family got married, and had a daughter. I recreated them for Sims 4, and I am trying to pair up the same two that got together in Sims 2, and see what they get. Also, the mother of the Tiger family got married, and she now has a 6th child.
I am slowly am remaking some of more memorable Sims 2 families into Sims 4, and giving them their own save file. I brought back my Leopard family, which is made up if a mother, her daughter, and her sister. They are memorable to me, because in Sims 2, the oldest son in the Tiger family met her while he was a College student, and she was an adult. She rejected him every time he tried to flirt, but once he graduated, she accepted his advances. He had a child with her, and become the stepfather to her daughter. I also had them adopt dogs as well.
Another way Sims are memorable to me is another way I make them. I tend to focus on series I like. So, if I like Harry Potter, I create it in my game. I would make some canon families, but also switch it up, like in the Harry Potter file I am working on, Lily and Petunia are living together with their families, which would make it kind of an alternate universe.
I have a Frozen save file, which kind of has the game play that is memorable. I have canon, semi-canon, and alternate universe versions all living in the same save file. It is funny, because it isn't unusual for one Anna to be friends with another Anna. Also, my Elsas all hang out together in the Magic Realm. The humor makes the whole thing memorable to me. Also, the save file won't stay a Frozen one, I plan to eventually turn aging on, they will end up ancestors to any other characters I plan to add.
I started a harry Potter idea (3 families so far), and I plan to create a Fruits Basket version as well for their own save file.
Smaller series are mixed in two save files with normal Sims I create,