The toss up for me is between Maria and Rachel.
Despite the value of her medical knowledge, her willingness to be post-apocalyptic "Breeding Stock" is questionable and risky if willing. Natural
childbirth without modern medicine is statisically risky. The odds of her clashing with any male in the three years are great.
Rachel is around 45+ as presented and makes her viability to have healthy children very low. This makes her the least valued member.
Chad's engineering skills and plannng are an asset thus given a pass.
Genetically speaking, provided there are enough males for diversity for the offspring. A one woman source would be enough to propagate the human
For repopulation purposes Cecil will have to be limited to two children with Brenda (hopefully female) and Chad will have to hopefully father a male
and female with Brenda. Ben, being the youngest male will have to wait until the Brenda's children are old enough if Rachel or Maria are not an
The largest risk genetically is Chad in this equation. If he is sterile then have a lower genetic stock and inbreading will have to be watch closer.
In any case Chad would be the planner if not actual leader of this inbread Brady Bunch, provided Ben doesn't kill him.