A tie between Cool kitchen (because of the already mentioned icecream) and Backyard stuff.
I rate the lemonade tray as near essential, because a sim can serve six glasses in one go, then I can drag one glass to each sim at the table for them to start drinking along with the meal. Without the tray everyone would have to get milk or orange juice from the fridge individually (or one sim up to eight times).
If you already have Spa day, it comes with a similar tray. In this case Cool kitchen is probably the better purchase.
