One way to make a sim memorable is to write a story about them. I have always played rotationally, aging on and have had sooo many characters. I think I remember most of my TS4 sims, so going to look back further.
I remember some of my TS2 characters, but their names are escaping me... I had a sim called Julius who had fairy ears and always wore a top hat. I believe he was in the politician career. One of my sims was called Anakin Sywalker and his face was so hideous that I had to use the career reward to fix it. One of my favourites was Sylvia Von Fieant. She had a romantic aspiration, I believe Anakin Skywalker was her son actually. Sylvia was the third child of the family I first created. I also played her siblings Lemmikki and Lenni a lot and they both had kids too. Lenni married Iiris from my other played household. I'm not sure if they had more kids, but I remember twins Amy Lee and Constantine who I've also recreated in TS4.
So yeah I remember some from TS2. Then going back to TS1 it gets even harder to remember names. I have recreated three of my sims who used to live together Elena, John & Claire Flowerhill. Elena was very artistic sim, John was a police and Claire was a thief. I thought it was so funny to play them all in the same household. I believe I also had a sim called Lena or Leena who wore the blue pigtails, not sure why she popped into my mind. And of course my first sims Petteri and Maria who later also had a child whose name I don't remember.
Back to the question. From the earlier games I remember best the sims that I created first and their family lines. I also remember sims who had some sort of weird quirks. I remember sims who I played a lot or who went through big changes. Names are hard for me to remember. I think having positive or negative emotions towards a sim makes them more memorable, but I must have forgotten many great characters. For some I remember the house where they lived and the household vaguely, perhaps even some family dynamics or general idea behind them, but then I have no idea of their names or what they looked like.