Self-defense classes should be chosen based on the practicality of fighting methods for combat. While there is nothing intrinsically wrong with traditional martial arts, such as karate or tae kwon do, many schools focus excessively on form or competition-based sparring.
Practical fighting maneuvers tend to be the simplest ones. In a fight, the adrenaline rush overrides fine motor function often necessary for complex maneuvers, such as the joint locks in aikido. Conversely, it's not always a good idea to focus on martial arts that teach exclusively debilitating or lethal combat moves. Not only should technical practicalities of moves be taken into account, the legal consequences should be considered. Few situations require lethal force, and excessive force could create legal complications.
Not only should a good self-defense class teach fighting techniques, it should also teach environmental awareness and combat de-escalation; in other words, violence should be emphasized as a last resort.
Prospective students should sit in on several classes and watch carefully to determine what class best suits their needs. Sparring should be emphasized and a variety of situations should be covered, such as being approached from behind. A good instructor also teaches the proper use of self-defense tools, including pepper spray and tasers.