While I'm no martial arts expert, I have dabbled in a few. I think that, generally speaking, if you're primary interest is self defense, Krav Maga is probably going to give you the most bang for your buck. I trained in it back in the 90s, and the techniques are designed to work with your natural instincts, which is one of the reasons why I think it can give people a basic level of readiness fairly quickly (weeks/months rather than years). Also, many of the schools I've seen often incorporate some form of groundfighting, which is valuable. Though I agree with those who said you don't want to take a fight to the ground. Rolling on a mat is one thing, but doing it on a gravel lot with broken glass and other debris is something else entirely. Not to mention it leaves you vulnerable to your attacker's buddies coming in and stomping on you.
While I haven't trained in it personally, I do like what I've seen of Systema.
My personal opinion is that most any martial art can be an effective form of self defense, if you train with that purpose in mind. Of course, some martial arts are going to be better at that than others. If I wanted to be able to defend myself, I would choose Brazillian Jujutsu over Tai Chi or Aikido.
Speaking of Aikido...
It was actually created in the 20th century, 1920s or 1930s if I'm not mistaken, by Morihei Ueshiba. However, it was based on Daito-Ryu Aikijutsu (or Aiki-Jujutsu), which Ueshiba had trained in. That art came about sometime in the 1600s or 1700s, I believe, and based its movements on sword techniques, which the samurai already knew. The idea was to give the samurai the ability to defend themselves should they be disarmed of their swords in combat. I studied a related art, Hakko Ryu Jujutsu (instead of using the attacker's momentum and circular movement in Aikido, it relied on throws, joint locks, and pressure point manipulation to defend oneself without causing injury to the attacker), that was also based on Daito-Ryu Aikijutsu. My instructor often told us to imagine we were holding a sword while executing the techniques.
And one thing my instructor said, which can also apply to Aikido, is that while the goal is to not injure the attacker, that's often likely to happen when the attacker doesn't know how to "receive" the technique. One of my classmates used a technique called Kote Gaeshi, I believe, to defend himself while working as a security guard. The attacker comes in with a knife and the defender steps to the outside, grabs the wrist, and, using the attacker's momentum, twists the wrist while turning his body. In the dojo, and on the movie screen (Steven Seagal has used this technique many times), the attacker then flips through the air, landing on his back. In this real life incident, the attacker didn't flip through the air, but crumpled to the ground with a broken wrist. Our instructor often taught us "street" versions of the techniques that were simpler and easier to apply under stress.
FYI, I've since changed my philosophy regarding self defense. While I think there can be some advantage to being able to defend yourself without injuring the attacker in certain circumstances (mostly situations involving EMS, LE, security, etc.), I think simple, straightforward techniques that are easier to learn and apply under stress, and that can cause an appropriate amount of injury/damage to an attacker, is probably going to be the best approach for most people. While pain compliance techniques can work, it depends on how the attacker perceives pain. Physical damage is probably going to be a more reliable means of stopping an attacker.
Just my opinion.