What is the function of 了 in the sentence 我最怕去西单了?
In a phrase with this construct:


  可/太/最 + [adjective/adverb/stative verb] + 了


了 serves as a modal article (rather than tense particle) to express emphasis and is optional.

Reference: http://cdmd.cnki.com.cn/Article/CDMD-10475-1012379314.htm