While Pets is my absolute favorite, mods, I hope this isn't a frivolous answer, but I think they're all worth it, in their own way! It all depends on what you like with your games. If you are big on raising families, I would recommend Generations. If you're into vacations and adventure, I would go with World Adventures of course! If you like the feel of big city, entertainment, celebrities, and Vampires, I would go with Showtime and Late Night. If you like Sims for the careers, I would recommend Ambitions. If you're an animal lover like me (I'm mostly a cat person), Pets is the most fun!
It's all based on your personal preferences. I can't just say one specific one is worth it, because they all are. It's all on your personal taste. Have you played Sims 2? What was your favorite EP from TS2? Maybe that will help you out, a bit as well.
Pam is right, you should look into Carl's guides. They're really helpful!