My favorite, must-have expansions are Seasons and Generations! Seasons and weather is just a part of life and so should always be included in a life simulator. I just love watching the seasons change, the rain, the fog, the heat waves, etc. And Generations is great because it greatly expands the family aspect of the game. More things to do for kids and teens and making them feel like fully-fleshed out life stages. I'm just really into that stuff.
My least favorite would be Into The Future. I simply don't play with it's features at all. Even with some of my least essential packs, I still use build/buy mode objects and CAS items. Not at all with Into The Future.
