Wow! It's tough to pick just one - there are so many.
However, one thing that's high on the list is time - time spent as a family building memories and really getting to know each other. That's not to say that my kids don't bicker and do things to annoy each other, but they will have shared memories of some good times together when they're out on their own,
Time factors in in other ways, too - more free time when you're working with only a few; more time to go places, take field trips, and experience real-world stuff; and the kids have time to pursue their own interests, as well.
Thanks for asking!