Emiya Montgomery.
I created her in The Sims 2 and did so much of the game content across all packs with her. There are versions of her now in both the Sims 3 and Sims 4.
Jen Li.
I created her and tried to do everything there was to do in the Sims 3. She doesn't have a version yet in Sims 2 or 4 but I played her the longest. I remember travelling ALOT with her and unlocking the different travel destinations. She had a home in all of them decorated with items that fit the destination - right down to what she had planted in her garden.
The Sandrin Family - Diena (mom) and Tiena (daughter). My very first characters ever created when I discovered The Sims 2. I was horrible at it as I had no idea how I was supposed to meet their needs so Tiena didn't have very many good memories growing up. She did meet a girl by the name of Maria Rizotello though and her mom, Angela. They all became the best of friends which has carried over into the Sims 4. It's weird but it's almost like I am trying to make up for those mistakes because now I try to make all four of those women's lives as perfect as possible in the Sims 4. I haven't created them in The Sims 3 (yet).
Jylia Michensen - My second household in The Sims 2 and my first household ever when I started playing the Sims 3 and Sims 4. She is the reason why I play with aging off. I always regret losing that particular household due to her unexpectedly dying - playing the household with just her wife wasn't the same.
There was another one that I was playing and I remember her because of her pets and where she lived. But I lost her when the computer I was playing it on died. I still have the computer though, it just hasn't been a priority to try and retrieve the data. I just like to think she has gone on permanent vacation...
So for me, what makes a sim memorable is the time I spent playing them, their house, pets or traits, or the things I accomplished with them.