It's tough because most of the expansion packs were filled with content and had great gameplay. If I had to choose, I'd go with Seasons. Closely followed by Pets. Both of these packs added so much content, I could literally play the game with just these 2 packs only.
My least favourite is between Showtime and Generations. Showtime got boring pretty quickly for me and the world wasn't great. Generations I know controversial but I never really got into it, for me it didn't add much to my gameplay and a lot of the features felt like they slowed me down, I get that's the point of the pack, to embrace the moments of your sims lives but it's not for me.
