Any Martial Art will have a positive impact on your life and your health. It will get you moving, and get your blood flowing. It is excellent exercize and will help you live longer. In an emergency situation, it will also give you the strength and the focus to see yourself out. In a self defense situation, you get the ability to protect yourself.
In my opinion, the best Martial Art to train is Brazilian Jiu-jitsu. It by far will give you the strength and the confidence to protect yourself if the need arrives. Boxing is also an excellent choice, as is Krav Maga. The key to picking the right Martial Art is to find a school where you fit in. If you do not like the school, you will have zero motivation to attend. Where I live, I practice BJJ at Black Hole Jiu Jitsu. I find that their training methods and the people at the school are all top notch. Just showing up is a big step, but the people there will challenge you in all the right ways. Give it a try. You really don't know what your missing, or what your like could be like.