Actually, the least restrictive constitutional test is rational basis.
Yes, least restrictive for the government.
Almost anything can pass rational basis. The government just has to make a claim that there is some rational reason for the law. It does not have to be true.
In several circuits, “Intermediate scrutiny” has collapsed to “rational basis”.
But, I am not a lawyer. I am willing to be educated to a different understanding.
Rational basis is essentially being able to articulate a legitimate government interest and stops there. Intermediate scrutiny steps that up to an important government interest and requires that the government action must be through means that further that interest. Intermediate scrutiny is commonly used for sex discrimination. Strict scrutiny requires a "compelling government interest" with the government action narrowly tailored to achieve that interest. Strict scrutiny is applied when basic rights are involved - speech, religion, etc.
This online legal dictionary runs through each of the review levels with some good examples.
Legal Dictionary Article on Strict Scrutiny