There are no best questions. In every situation they are different. Limiting yourself and a candidate only to a single technology is a bad practice, moreover, it is something a good engineer can learn fast. Fundamental knowledge is what you should check for, and personal skills, of course, too. I am always asking:
- Describe your working process, software development lifecycle, your environment, tools, what do you use, what do you start from, what is next and what you end with
- To show problem solving skills and analytical thinking by giving a simple task
- Software engineering, programming languages, software architecture, design patterns, basic math and algorithms
- Team work, public activity, leadership, maintainability, security, integrity
- About a web standards, web technologies, HTTP, trends and news
- Hobbies, other activities, social, business, communication skills
- Motivation, past experience, future vision, what didn't liked, what would like to change, why
- Teach me something
Talking about a beck-end/Node.js I would ask what a person knows about it, what languages and frameworks can be used for that job, which one he or she prefers and why. How Node.js is different from others, what some good practices, use cases or failures he or she knows. Since Node.js is a JavaScript, some JS test should be included also.