Choosing a Martial Art to practice is most certainly the first, and perhaps the most important question we ask ourselves before beginning the journey. If we are to base the question purely on the merits of which one will suit our needs best, the answer is fairly straightforward. If self defense is your top priority you want a "soft" or "internal" Art. If that was what I was looking for, JuJitSu, Hapkido or Judo would most likely be the top three. You get to be Stephen Segal and learn lots of "no nonsense" techniques that work well on the street. If, on the other hand, you want to be Jean Claude Van Damme and have an arsenal of impressive "big" techniques you want a "hard" or "external" art. Taekwondo, Karate or Muay Thai will then fit the bill nicely.
There is, however, a more important consideration and that's the quality of the club or school you decide to enroll in. If you pick a bad one, you're in for a bumpy ride. First problem will be cost - bad schools/clubs will continually allow you to grade in order to collect grading fees from you. Don't be naive - there are bad clubs and bad instructors out there with a main focus on how much money they can get out of you before you (inevitably) quit. I attended a Karate class once that consisted of nothing more than the instructor showing us (we were all beginners) a long demonstration of advanced techniques. We didn't actually do any training, and I never went back.
I'm in luck here - the club I train at has a Master who's integrity, skill and passion for the art are self evident and his classes are run by instructors who share the same passion for the art. Taekwondo "does it" for me because so many things I want can be achieved. My top priority is to maintain fitness and flexibility as I move into old (or older!) age, but I know I'm getting a healthy dose of self defense (we get taught Hapkido moves as part of the syllabus) and the sport aspect of Taekwondo gives me the chance to try out what I learn in as real a situation as possible without actually (most of the time) hurting anyone.
The short answer to this question - Taekwondo is without doubt the most rounded Martial Art and if you can find a good club it will serve you well.