Is at the least cathartic a sentence?

No, "at the least cathartic" is not a complete sentence. It is a phrase that acts as an adjective phrase modifying a noun or pronoun.

To make it a sentence, you need to add a subject and a verb:

* Examples:

* "The movie was at the least cathartic."

* "Writing in my journal is at the least cathartic."

Let me know if you'd like to see more examples!