As already mentioned, it depends what you wnat to do and what you want it for.
I have boxed in the Army, had unlicenced boxing fights and have had over 30 Full Contact Professional Kickboxing bouts. If you are looking to compete in full contact I would look at either boxing, full contact Kickboxing or Tai Boxing
If you want something that will keep you fit, learn self defence and allows you to gain gradings etc then I would choose either Karate or Kung Fu. If you want something for the street, look at Mixed Martial Arts as they incorporate everything.
The most important thing is to find a club that has a decent set up and an excellent Instructor. There are to many instructors now who get a Black Belt and decide to teach just to make money. They have only been training a few years themself and know very little.
The Club I trained at had Full Contact champions from National to World Class and none them had any sort of grading belt. We were all just equal. My advice is to decide what you want, find a decent club which can be usually be told by the caliber of its members/Instructors, then join and enjoy.
Hope this helps