I like the ones with a little bit of added game play:
- Little Campers - you get the don't wake the llama/bigfoot game even without Get Together, a fire pit and cooking s'mores on it without Outdoor Retreat or Horse Ranch, a dollhouse that is relatively cheap and since it's made from cardboard boxes actually makes sense being the size that it is, a big projection screen without Movie Stuff, a swing set without any of the expansion packs that include them, and a relatively cheap kid's activity table.
- Riviera Retreat also has a fire pit if that's what you're looking for, though if you're only getting one kit I'd still go with Little Campers.
- Book Nook has a lot of objects that you can use to unobtrusively place books in ways that they will actually stay when you save the lot to your library (unlike books you place inside bookshelves) which I like for custom library collections or reference books on public lots.
- Storybook Nursery has a rocking chair and changing table.
- Casanova Cave has another giant screen, though again if you're just getting one kit I'd go with Little Campers. Vastly different looks though; Casanova Cave's would not look out of place in a movie theater build.
- Kitchen Clutter has the grocery bag you can order groceries from (if you have Cottage Living & like playing with its Simple Living lot challenge, this will be wonderful for playing with both Off the Grid and Simple Living together). It also has a lot of fun touches like getting cereal from the cereal boxes and cookies from the cookie jar.
- Restoration Workshop has the appliances that can be purchased broken and restored in order to use them.
- And of course there's the very first gameplay kit - Bust the Dust - but I haven't actually bought that one because I have some concerns about how it integrates into the game (mainly, that there's no broom; I also believe both the dust bunny gifts and moodlets were overpowered originally but they may have been toned down since).
I've also found I use the Grunge Revival CAS items frequently and the Cozy Bistro flooring all the time.