I don't have a lot of packs, but I think Get To Work, Get Together, and Seasons have a lot of potential for player creativity and can serve as solid foundations as you build up your worlds/stories/playing style.
With Get to Work's retail feature, you can think up basically any type of store you would like to own in the game (and the more packs you add afterwards, the more objects that become available, which means the more kinds of stores you can have). This could help with world building as you can place all kinds of shops in your game.
Get Together's club system allows you to create clubs which allows you to influence which Sims interact with one another, and in what kinds of ways. Again, the more packs you have the more kinds of activities the club members can do. I've read about a variety of clubs from skill-building clubs, to social clubs where people use the club feature to keep families connected with one another, and all kinds of clubs for whatever kind of storytelling the player needs. If you have Bowling SP you can have a bowling club, if you have ROM you can make a coven of witches, if you have Seasons you can make an ice skating club or a gardening club. Whatever you can imagine up.
Aside from the seasons/weather, Seasons also adds a holiday feature which you can edit to make your own holidays, which adds to world building.
I think these three packs specifically added at least one feature that basically allowed for more immersive integration with other packs which makes them more dynamic and allows you more customization in your gameplay.
