Obviously, the most important part of MySims is the Sims! Each NPC sim in MySims features completely unique dialogue and very cute designs. I find all of them delightful, I don't really dislike any of them.
Here's some of my faves!
My reasons for loving Beebee should be (be) obvious. She's pink. She's a bunny. In fact, in her MySims profile, she is explicitly the bunny princess! Don't tell anybody, but when I was a kid, I used to pretend to go on dates with Beebee :')
My favourite appearance of Beebee is in MySims Agents, where she's the ditzy girlfriend of water bottle billionaire Preston Winthrop Esquire. I have a real soft spot for dumb girls in love, and their dynamic is sad but hilarious. She also gets an absolutely iconic snow outfit.
DJ Candy is basically a mascot of the franchise, and it's for good reason. Her colours are so bright and fun, she has great, unique hair. She's also pretty gender-neutrally cool, to the point where I thought she was a boy as a kid.
Personality wise, I really like the slight complexity that Candy has. She is incredibly intelligent, for example, having a lot of technical expertise with music. She's also one of the only sims to have spoken English. Specifically to Dead Space's Isaac Clarke in an advert for MySims SkyHeroes. It was wierd.
Brandi is another sim I would go on pretend dates with as a child (allegedly). Brandi's whole thing in the original MySims is that she lies. Although, as a townie she doesn't get much dialogue, so I mostly just loved her aesthetic as a kid, especially the hair.
Brandi gets a huge glow-up in MySims Agents, though. She gets a job at MorcuCorp and basically forms a Team Rocket situationship with Esma and Yuki. Her dynamic with her co-workers is really campy and fun, and it's a shame they didn't continue it in MySims SkyHeroes.