Laundry Day has to be my favourite SP. I am also a family player and I wouldn't want to play without it now. There is just something very satisfying about watching my sim families do their laundry. If you have Parenthood, it also helps to raise child sim's responsibility. It is one of the largest SPs with lots of great CAS and BB items, as well as gameplay value.
Toddler Stuff would probably be my second choice. If you like to play toddlers, it gives them some much needed cute CAS items and activities. I enjoy taking my toddlers out on play dates to toddler parks.
Backyard Stuff is another of my favourites. I love the CAS and I use the wind chimes, drink tray, and bird feeder in almost all of my family homes. I also think it's great for families if you like to play outdoors and it goes well with the Seasons EP.
I also really love Vintage Glamour, which works well with rich or famous families with a butler, lots of great vintage style CAS, and BB items like the bar globe and vanity table. Child sims are able to study the bar globe and play with make-up which is really cute.
Movie Hangout has some great boho style CAS. I like watching my sim couples cuddle up to watch movies and eat popcorn. Cool Kitchen has some nice CAS too and the ice cream machine is good for families, particularly if you have Dine Out and want to take the kids out for ice cream. I didn't think I would like Bowling Night that much since bowling isn't my thing in real life, but I actually enjoy taking my families out bowling and the retro style CAS is great. I don't use Spooky Stuff much, but it is perfect for Spooky Day if you have Seasons. I have mixed feelings about Fitness Stuff since I was disappointed we didn't get exercise bikes or dumbbells, but the earbuds and workout videos are nice. I don't have Moschino Stuff yet, but from what I've seen it really improves the photography skill and will be useful for family photos.
Anyway, I hope that helps at least a little. I recommend checking out the SimsVIP guides and reviews too if you haven't already to see which SP appeals to you the most.