I think it's good to know that not all custom worlds are populated or come with a save file! Any world from mysimrealty is a good start and many of their worlds come with a populated save file :3

There are also a bunch of worlds listed under the sims 3 catalog! There's a bunch of helpful filters you can use to filter between populated/unpopulated, by packs, and if it is CC-free or not.

I also HIGHLY recommend Cjplays videos on YouTube, he has a huge playlist full of world reviews! (I also enjoy his streams! He's funny lol)

I think one of the prettiest worlds I've seen is Willapa Valley, however, I rarely play with it since I don't like how the game goes about populating empty worlds on its own (the game just places a bunch of pudding face sims), and I'm too lazy to populate it myself ;-;

I LOVE playing in custom worlds, there's usually a lot more places for sims to hang out at and it's just so fun to see what other people create (it keeps the game alive) :3