I like a lot of the music related animations watching sims be so terrible and eventually get better with it. Especially with singing. There singing is so bad. Though most of the animations generally stay the same I like hearing the change in their voice.
Also the emotional changes in voice and animation as well is another favorite. There's so many animations I really like in this game hard to narrow them down.
I also like seeing children drawing at the activity table and with varying animations depending on their mood.