Answer:
What is "best" is entirely a matter of opinion. What is effective, and works for self defense is available in many forms and various system of Martial Art from all over the world. What makes it effective to a great degree depends on the feasibility of the technical applications, and proposed tactics, as well as the skill of the individual. This will be determined by the competency of the teacher, as well as the dedication, and successful application of the individual's abilities.
Most experts would probably agree that the best type of self defense is avoidance. These means that you keep alert to potential trouble, and avoid situations and locations that would likely be hazardous. It also means that you check your attitude, keep your mouth shut, and avoid getting into fights whenever possible. Many people who get into fights went looking for trouble in the first place. Another good aspect of self defense is being physically fit. You don't have to be the strongest person in the fight, but having good stamina, and toned muscles helps to apply techniques with better skill, accuracy, and self control.
Perhaps the best advice is simply to look around at various schools of self defense, and find one that has a good reputation, qualified instructors, good quality students, and that you are comfortable at where you can learn, and are going to stick with it for a long time. While a person could study the practice of using weapons for self defense (mace, pepper spray, sticks, knives, guns, etc), knowledge of how to use your body as a weapon is something that is always available, and no one can take away and use against you. The three main areas of unarmed combat include skills of 1. striking, 2. Throwing, and 3. Grappling. Some systems have a preference and priority for long range striking, others go for close quarter strikes, throws and holds, while some systems focus almost entirely on ground-fighting and grappling. The choice is yours.