"Swimming is one of the healthiest exercises."
In the above example if SWIMMING is subject please let me know what grammatical label can be given to the word "exercises"
It is a plural noun.
It is part of the phrase "one of the healthiest exercises", which is the subject complement of "swimming".
Within the phrase, "one" is a noun, which is modified by a prepositional phrase "of the healthiest exercises". So "exercises" is the object of the preposition "of".