"The biggest spenders are we students who are interested in fashion."
Here's why:
* "Us" is an object pronoun. It's used when the pronoun is the object of a verb or preposition. In this sentence, "us" is the subject of the sentence, so it needs to be a subject pronoun.
* "We" is a subject pronoun. It's used when the pronoun is the subject of a verb or clause.
* "Are interested" is the correct verb conjugation for a plural subject ("we").
Here are a few other ways to phrase the sentence:
* "We students, who are interested in fashion, are the biggest spenders."
* "Students interested in fashion are the biggest spenders."