Final answer:
The main subject of the sentence is 'A piece of chocolate candy'.
Step-by-step explanation:
The main subject of the sentence is A piece of chocolate candy. This is because it is the noun phrase that the sentence is primarily about. It is the subject of the sentence and is followed by the verb would taste and the adjective great. The other options, Taste, Great, and Chocolate, are not the primary subjects of the sentence but rather complement the main subject.