Dialogflow Enterprise Edition is the optimal choice for this scenario due to its comprehensive capabilities in managing conversational interfaces, including voice assistants. It excels in intent recognition and conversational flow management, essential for a seamless voice assistant experience. While options like Speech-to-Text API and Cloud Natural Language API offer valuable services such as audio-to-text conversion and natural language processing, respectively, they lack the integrated conversational management features. Similarly, AutoML Natural Language, though powerful for custom NLP tasks, does not provide the necessary framework for building a full-featured voice assistant. Thus, Dialogflow stands out as the most suitable solution.