I've trained in Xin-Tao Karate for just over 2 years and earned my 1st degree black belt; then I effectively started over training in Olympic Tae Kwon Do for 3 years until I earned my 1st degree black belt in that art as well. I've also trained in Shotokan Karate, Filipino Escrima, Jeet Kune Do, Wing Chun, Bo Staff, Tai Chi, Juijitsu, and Krav Maga.
I teach a blended art of my own creation based on most of the above styles but mainly Karate, Escrima, and jui-jitsu.
I believe that if you're looking for real world self defense- other then my school- Krav Maga or Kajukenbo is a good place to start; if you're wanting more social aspects such as tournaments or you like Tradition then arts like Shotokan Karate, or Olympic Tae Kwon Do would be good fits.
In every art you'll learn skills that can be applied in real life combat, even if you have to sort of read between the lines as it were.
I'd stay away from schools like ATA or other schools that ask for hundreds of dollars upfront, charge more then $120/month for training, or have something like 20 belts that each have a $50 fee; there's good schools in these organizations and some solid skills but- at least for the schools where I am- i get the impression they are in it for the up-charge and such with a view of student numbers as money and not as individuals.
Find a style you like; research it; ask questions and weigh pros/cons and as long as you're learning something that you value you should be fine.
I have had 5 different instructors over the years; they all taught me some things of value; they all had stuff that I probably won't teach to my students but see the value of those drills from within that style. If you find a valued instructor who builds you up, keeps you humble, and doesn't treat you like an ATM; you should be fine. And (Shameless plug) if you find yourself in the Portland Oregon area; I still teach classes every week.
Where you put your effort is where you'll get your results. Train hard, ask questions, stay humble.
Hopefully that helps,
-Blackbeltmitpen
