Superb (and a warning to users)

Full-featured, flexible and reliable. Especially useful for a course setting because of the variety of ways you can give users feedback after they take the quiz. You could easily pay for a plugin that doesn’t do half as much.

The warning:

The quiz configuration page is laid out in tabs. You must save the contents of each tab before switching to a new one. This is especially important on the ‘Questions’ tab. You add questions, and you get a little message saying ‘Question saved!’ for each one – and your questions are not saved until you click the ‘Save questions and pages’ button. Until then, switching away from the ‘questions’ tab will delete all your work.