It might be difficult to determine what is the most popular world wide, but several stand-outs include Taekwondo (gaining full Olympic status in 2000), Judo (already an Olympic event), various forms of Karate, jujutsu (and various forms including Brazilian, Kung fu, and capoeira. Some people consider Boxing and Wrestling to be in the Martial Art category, but certainly not by the widely accepted philosophy of combat warrior training combined with mental and spiritual development with a warrior's code as a way of life.
Some might think that the most popular martial art is MMA, or "Mixed Martial Arts," however that is not an established Martial Art in and of itself. It is a "concept" of blending fighting skills, tactics, and techniques borrowed from a variety of recognized Martial Art systems. MMA might be popular as a sport, but even sport Taekwondo, Karate or judo is not the Martial Art itself. They are games played under rules, and help to make the actual Martial Art they represent and come from more popular and well-known. MMA doesn't come from one Martial Art system, and neither does Bruce Lee's Jeet Kune Do. They are concepts outside of the standard Martial Art curriculum.
This is a very subjective listing and there is no 'correct' answer. Lots of martial arts are effective, if the individual has learned and practiced them. They are all 'reliable' depending on how you define reliable.
One answerer viewed a number of sites and came up with this list.
1.Jeet Kune Do
2.Ninjitsu
3.LOTUS self-defence
4.Aikido
5.Krav Maga
6.Jiu Jitsu
7.Judo
Note that the list above does not include any 'striking' arts such as karate and boxing, which are the most common martial arts and are all highly effective if one trains them for self defense.
It depends on the situation. It depends on what you are doing with the Martial art. Say you are doing grappling on the ground the ones that do a lot of grappling like 6 and 7 would be best. The standing fighting styles are better if you aren't going to the ground. The absolute best combination is a standing fighting style and a grappling fighting style together. If you went by UF bouts a while ago Jiu Jitsu was on top but not some combination of styles have proven to be the best.
A martial art is only reliable if you don't meet someone better than you. There are lots of them. This includes just nasty, untrained veterans of years of bar fights who are drunk or just feeling unhappy.. This makes the mystic art of staying alert and avoiding bad situations a real plus. There are also skills like never insulting the meanest guy in the room and not saying the words "You'd better watch out - I know (insert name of your martial art)"
the most violent form of traditional martial arts is street fight and taekwondo.
ken-do in a form of martial art that uses primarily swords, although most martial arts use weapons at some level.
Most likely Taekwondo or Karate, these martial arts have many families involved and most studios of these take both children and adults as students, compared to martial art styles like BJJ that may not.
Karate
If you're asking where martial arts was born ... Then most believe it was born in China , and that , " Kung Fu" was the first martial arts form.
The origins of martial arts a shrouded in history. It is believed that most of the arts can trace their history to the Shoalin temples in China. They were Buddhist. As the arts spread, most of the religious connections were stripped off.
Martial arts are no more evil then a gun or a knife, or even a stick. In fact, most true martial artists are recognized more for their contributions to society then their fighting ability. A true martial artist would never consider attacking someone for fun or just because they can. a true martial artist understands martial ethics.
in most martial arts it takes about 4-6 years to become a black belt
It isn't.
You most certainly could. The martial arts tend to have some pretty good move and pops. Many martial arts demonstrations are set to music. yes
The can be found in all races throughout the world in all locations. Most of the world has combat skills that they pass down, the martial arts.
It depends on the specific martial art. Most of the arts will teach something along the lines of defense against weapons.