I have seen both forms, so I am confused. Which of the following is the most appropriate way to write the phrase?
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Is the expression used as a single adjective before a noun? If so, use hyphens (see example 1). For example: "He is a one-of-a-kind man."
Is the expression used as a predicate adjective after the verb to describe a noun before the verb? If so, don't use hyphens (see example 2). For example: "That man is one of a kind."
The hyphens' function is to join two or more words to show that they belong to each other.
See this example in Oxford Dictionary for Advanced Learners "a non-native speaker of a language is one who has not spoken it from the time they first learnt to talk", where non-native is an adjective.
To better fix the use of hypens for phrases being used adjectivally, read the sentence below:
A knave, a rascal, an eater of broken meats, a base proud, shallow, beggarly, three-suited, hundrea-pound, filthy-worsted stocking knawe; a lily-livered, actiontaking, whoreson glass-gazing super serviceable finical rogue, one-trunk-inheriting slave.
Here, certainly Sheakspeare would been able adding one-of-a-kind (followed to other word ... !).