When drafting an answer to a civil complaint, you should assert all relevant affirmative defenses. If you don’t, the defense is usually waived in federal court and many state courts. Of course, waiving a relevant defense may subject you to an ethical complaint or a malpractice claim, or both.
One good method to determine which affirmative defenses to raise is to categorize the available defenses. I have listed nine categories below, as well as common affirmative defenses for each category.