I really just wanted to take a moment to thank everyone in this thread for the recommendations! My favorite world has always been Sunset Valley. This is probably (in part, at least) because I started playing The Sims 3 when it first came out and it was the only one available, lol. I just feel HOME there. I recently bought Monte Vista and it looks absolutely spectacular(!), but I haven't really tried it too much yet. I could never get into Riverview and Hidden Springs (which came with Monte Vista) doesn't look all that appealing either, but maybe I just need to spend more time in them. Someday, perhaps. With each world being so customizable, I could probably spend years in any of them.
I'm currently just getting back into The Sims 3 after many years of neglecting one of my favorite games of all time. I bought all the expansions I didn't already have (everything post-Generations) and am overwhelmed with content and worlds to try.
I wish I could add my own recommendations, but every user-created world I've looked at (and then some) has already been mentioned here. I haven't tried any of those yet (ever), but I was already leaning toward getting Legacy Island 3 and this thread pretty much cemented that one for me.
Thanks again everyone!