"most foundational" is correct and usable in written English.
You can use it to emphasize the importance of something that provides a solid basis for something else. For example, "The most foundational concept in linguistics is syntax."
And the redemption narrative, as Dee clearly realises, is the most foundational American story.
The internet isn't the most important fight we have; but it's the most foundational.
While all of these qualities surely correlate, scientists do not truly understand which is most foundational.
This is why Mormonism places in the hands of high schoolers the most foundational ritual of Christianity.
Among the many emotions and feelings that tie us to brands, trust is one of the most foundational.
We, in some ways, performed an intricate reading of one of the most foundational works in the western philosophical tradition.
"The most important foundational work that we do is monitor domestic criminal trials.
It also unnecessarily compartmentalizes the most important foundational knowledge base of modern healthcare.
Right now, you or I could start trolling some of the world's most famous, foundational research and connect the dots to subjects and their family members.
If anyone thinks this sort of intellectual midwifery by metaphor is a thing of the past, they need only consider whether one of the most familiar foundational physical theories currently in play: string theory.
The fact that the weight of the evidence, as presented by Mr. Finkelstein, goes against a belief in one of Judaism's most important foundational stories will no doubt be disappointing to some readers.
