Hi O, I'd vote for adverbial. For one thing, the phrase does not equal Nora , which condition I believe is necessary for an appositive. If you rearrange the sentence (if this is allowed) you have, Nora is the best of all the candidates who are running , and (I think) the phrase may be said to modify best , which is a predicate adjective (I guess you call it complement ), so it takes an adverb to modify an adjective.
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Is "of all the candidates who are running" an appositive or some sort of strange relative/adjective clause.Not an appositive. Not a strange clause either! Just an ordinary prepositional phrase.