Here's your answer:
Most boxing gyms do a non contact fitness class, sign up to that first for a few months to get your cardio, coordination and stamina up to par then after you've learned the basics like footwork, guard, jab, cross and hook then upgrade to contact sparring. A decent coach will ease you into the sweet science with proper training and its by far the most effective self defense you can learn and the fitness benefits are out of this world.
Everything else is too flashy/showy, needs lots of room to execute along with loads and loads of training to get the moves as fast and powerful as possible in a combat situation. Boxing on the other hand can take an average dude and make him able to protect himself within a 6 months in pretty much any scenario as the videos show. Most people can't throw hands effectively and fewer still can evade and even less can counter punch.