There are three types of interviews: unstructured, semistructured, and structured.
Structured interview questions can be open-ended or closed-ended. In a structured interview, well-trained interviewers ask a set of planned, rigorous, and relevant interview questions and use a scoring guide to make sure their interview ratings are accurate.
So the question remains..... which style is best. Where a structured interview may seem like the best for understanding the candidates qualifications this can only happen if you are being interviewed by a trained employee.
The best course of action is to be prepared for all kinds, structured / unstructured or somewhere in between. Follow all of the steps for preparation to ensure a positive end result.