I can only 1 and two since I never bothered buying an expansion (i had one in my hand but i bought something else in radioshack instead)
1. Sunset Valley. Adding new worlds kind of lags my game. I had a few installed before but I always found myself rushing back to Sunset Valley. I guess its because i get a sentimental feeling from the first time I played sims 3. Another reason is the size and the number of large lots. Some of the other worlds have too much small lots and even when i manually edit the town and bulldoze the first building, I just feel like i dont have enough space for what i want.
3. Eight. The mother and father had three kids and when the first member got married, they had twins. After I moved them out into a good house. the second kid got married and had a kid and the parents had another kid, which lead to the thrid kid moving in his gf because he finally got moved into his older sisters bedroom.
So The family was alway at 8 until the parents got too old to have more kids and the youngest (who inherited the house) kicked everyone out after the parents retired.
Sorry, that was a mouthful. PS: Don't have an eight family household with pregnant and baby sims randomly around. No one gets sleep and the mother goes into labor at the worst time