This article rates the best combinations of children (for families that have more than one) based on children’s behavior, including ease of care, compatability and overall behavior.
The winning combination is two daughters. With a boy and a girl (it doesn’t differentiate which comes first) it looks like we have the second best combo. Although anyone with two kids is guaranteed one of the three top slots.
Jump from a boy and a girl to one more kid you suddenly fall from number two to seven or eight. Apparently there is a benefit to single-sex children among families with more than two kids.
Then again, this is looking at the best combinations in terms of the difficulty raising the kids. It would be interesting to see how the ratings change later in life. Are interactions more frequent, of a higher quality with adult children of a certain gender, of a certain number?