The sentence is an example of predicative nominative. The predicate nominate is “hitter.” The sentence is a bit tricky because the compound adjective “home-run” precedes the noun “hitter.” If you break the sentence down to simpler terms, however, the sentence is simply “Hank is the hitter.” “Greatest home-run” are adjectives that modify hitter. The sentence would be an example of predicate adjective this way: Hank is the greatest. Greatest is an adjective that modifies Hank. Generally speaking, a predicate adjective refers to adjectives in the verb part of the sentence. A predicate nominative, also included in the verb part of the sentence, features a noun.