That depends on what your product / service is and how comprehensive your help section is.
A list of the most common questions is great when there are a few questions that most people will want to know, but is a poor choice for complicated products. Imagine if the Apple used a list of questions and answers on for their help section. It would be almost unusable.
When you have a detailed help section covering many topics, it is generally more usable to have answers in sections. In this case, it is especially helpful to have a good search feature.
In the end you have to decide what is more appropriate given your situation as there is no one better solution for all situations.