This is harder for me to answer than I thought it would be, but here goes...
Favorite Expansion Pack: Seasons. It's hard for me to remember Seasons is actually an expansion pack, because it's such an integral part of all my saves. Having a calendar with changing seasons and weather make my Simworlds seem so much more real.
Favorite Non-Seasons Expansion Pack: Cats and Dogs. We're pet people at my house, so having cats and dogs was essential. Plus, Brindleton Bay reminds me of the New England coast. I have a lot of happy memories of time spent there.
Favorite Game Pack: Outdoor Retreat. I enjoy the way sending Sims on vacations changes the feel of a save.
Favorite Stuff Pack: Tiny Living. Even if a Sim isn't living in a tiny house, I find myself using the build and CAS elements of this pack very frequently.
I don't have one of the kits...yet.
