What single expansion pack is essential for fun playing TS4 and why?

If I had to pick an expansion, I'd probably say Get Together because of the clubs. You can basically make a gang or a cult and its very versatile.
What makes the game playable for me though is not what you mentioned but the vampires and spellcasters, and I usually have at least some kind of club for each. I know there's mixed feelings about occults but for me the more bizarre I can make my sims world the better.

At the end of the day its what you personally find fun. There's certainly different styles and things of playing.