I have practiced quite a few arts, and while I believe none of them will give you a complete arsenal, taking a few different ones can. The first choice when starting the arts is finding the right teacher, and the second is finding arts that promote relaxation while teaching the fundamentals which are, power comes from your hips and legs, and endurance is at least half of the battle. Good arts to take are Brazilian Jui Jitsu, Aikido, Kick Bboxing, Judo, Escrima, and specific forms of karate like Kempo, but not classical karate that teach tight memorized forms (Bruce Lee called "the classical mess") and where skill level is determined by how many boards or bricks you can break.
I think everyone taking martial arts, regardless of what it is, should also learn boxing. I have seen far too many martial artists in every art, who get their black belts and still cannot make a proper fist, do not know how to punch correctly, and do not know that their power comes from their hips and legs. Boxing will teach all of this. It will allow you as a martial artist, to be able to effectively set up throws and take downs, while giving you powerful knockout and finishing punches. That does not mean you have to go full contact. There are plenty of drills that can teach a beginner to be a devastating boxer against some street punk within 6 months without having to go full contact. I suggest getting the feel from time to time of a glancing punch say, so if it happens on the street you are not surprised.
Aikido can be one of the best martial arts to learn for street defense. It teaches relaxation, along with a flexible form that can allow you to defend against one attacker, multiple attacker's punches, grabs and kicks, knife attacks, sticks, bats, etc. To be truly effective on the street you should know how to punch effectively however, and that is something most Aikido teachers do not teach. For almost every throw or take down to be set up effectively, it is imperative that you are able to strike first, sometimes in between, and even after a throw or take down. Even a double or single leg take down works best when set up with striking. Then to top it off, if you are experiencing multiple attackers, you may want to break the arm or wrist to minimize further attacks. Boxing has a very similar stance as Aikido, with a slight difference in the rear foot position, and you can alternate between the two <myspace>style</myspace>s of footwork quickly and easily. Both of these arts should teach that power comes from the hips and legs from the ground. I suggest also learning to kick effectively and Brazillian Jui Jitsu to make a very well rounded arsenal.
Except for ground work, Aikido would be nearly complete if boxing was taught with it. These new black belts in Aikido think they are short of invincible, and think they can get by a street attack without punching skills. The fault is that of the teacher's for not teaching something so vital. If for some reason you cannot manipulate an attacker with movement and your atemi fakes, you can first smash a jaw, nose, eye, ribcage, solar e. plexus, and then continue to move him with your Aikido skill if he didn't already get knocked out or down. The most powerful Aikidokas I ever saw were experienced boxers.
Brazillian Jui Jitsu is a fantastic art for ground defense, but you need to learn stand up as well to have a complete arsenal. This art does teach stand up skills, but I have found that you either find teachers who favor teaching ground work, or you have to train for a very long time before you start learning stand up. In my opinion, no art will teach more effective striking in the shortest amount of time as boxing will. Brazilian Jui Jitsu is a good art, and with efficient stand up, you can defend against multiple attackers, and disable them quickly one at a time. That means you must know how to punch with power, kick, knee, throw, and if taken to the ground, be able to break a limb quickly and get up quickly. You must be able to move fluidly on your feet around your opponents as they attack, which is where Aikido training is beneficial.
Judo is good for ground fighting and stand up, but the way it is taught, it is mainly used for competition. It has many of the same techniques as Aikido, except they take them out because they are illegal in competition, due to the injury the techniques cause. The takedowns are similar to Aikido except Judo focuses more on driving an opponent downward while Aikido throws often are projected outward or up and into things. Judo throws are devastating because they drive you so hard into the ground. Aikido throws are more designed to break the wrist, elbow, and shoulder at the same time as you are throwing. Judo has some great trip techniques that when pushing a bigger man back while kicking them off balance, can land him on his head. I warn that in fighting competitions like UFC and defense on soft ground, a judo throw may not hurt an adrenalized attacker enough and can actually make them mad and more violent. I have seen tough street fighters beat the crap out of Judokas after getting slammed repeatedly in competition.
Kick Boxing is a very difficult art to get good at. It is probably one of the few arts on my list that requires some real pain and sweat, but if you can deal with it, it can make you a devastating kicker with speed, accuracy and power. I have seen light weight guys that can suspend a heavy bag up in the air while repeatedly kicking it exactly in the same place every time. It takes a lot of position strengthening for specific muscle twitch isolation, and it will leave you shaking like a leaf after doing that and pyramid count push ups, and then finally starting your drills. Kick Boxing is probably the only art that traditionally teaches Boxing. Some teachers will teach the actual boxing portion of the art after their student's have become efficient kickers. When I was taking it, the boxing was taught the same way as a boxing gym would teach it.
I have to admit that I am fairly biased towards classical karate. I have seen classes where people are merely jumping back and fourth while throwing uncontrolled kicks and really sad punches. Karate has been severely watered down in the states. Classes are often packed, and the classes are usually tailored towards keeping paying customers, while creating a workout that can be done by anyone. People don't want to really work for the knowledge, they seem to be mostly into gaining a new colored belt so they can brag. The belt system is how the teacher can hold on to a student longer. Then classic karate wastes students time teaching useless techniques like fighting with an oar, or a really long stick, or any number of things. Then you have the board and brick breaking which is none other than a trick. Sure it may teach a student the confidence to stand there and break a brick, but the bricks and boards are all strategically placed so as to break in a certain way.
Generally the idea is, if you go to a dojo that has a lot of people, it is watered down to make it easy to attract and keep the most customers. It attracts children mostly because it is more mechanical and easier to learn than other arts. I have seen grown men doing classical Karate Katas, and they are extremely tense, stiff, unbending, and seem to lack any intelligent thought for an otherwise potentially dynamic, ever changing street scenario.
The dojo's that have only a few students training, tend to be taught by a hard core traditional teacher that makes the students work for the knowledge. I took Kick Boxing at the Boy's and Girl's Club. The teacher had us doing 200 push ups as fast as we could count off, holding strengthening positions and drilling until we dropped. There were maybe 4 students that wanted it. The teacher allowed anyone of any age to train there from off the street, but few wanted it. My Aikido teacher taught Aikido along with Brazillian Jui Jitsu, boxing, escrima, kicking, knives, etc. The class topped out at 6 students.
Other Aikido teachers had 30 students, but they were given black belts quickly, and they couldn't fight their way out of a paper bag because the classes were so watered down and lacked vital techniques. Again, the teachers preferred to teach pretty, flowing, complicated looking techniques, that had little street application. That is what many people want to learn though. They are not learning a martial art for it's purpose. They are learning to dance, or just getting some exercise, which is fine, but should be separated so students are not getting black belts believing they know how to properly defend themselves on the streets, as martial arts were intended for after the war.
The last art I want to mention is Escrima. This is a Filipino martial art that was taught to Filipinos for fighting gorilla warfare in the jungle. They fought often with two machetes. For training we use really tough short bamboo sticks. You learn how to effectively strike, block, and take down with one sticks or two. The art has some devastating arm locks and breaks similar to Aikido and Jui Jitsu. The take downs and finishes are brutal and are for maiming, and killing. They also teach knife defense and offense as well. For serious offense/defense, this is a real effective martial art. It is also sometimes accompanied with other arts like Kempo Karate, a more fluid form of Karate.
My advice is if you see a teacher hurting their students in class, run away. You can accumulate a lot of serious injuries from teachers and over enthusiastic students with no control. If you want to continue training for defense when you are in your 60's and 70's, try working smarter. You don't need to do one hundred high falls in a row to get good at a martial art. People have killed themselves doing stupid things like that. Stay away from competitions for the most part. Most of us should be training to defend ourselves if we have to, and to maintain good health so we can continue training in our senior years. You are more likely going up against an attacker with no discipline, no endurance, and who is probably damn near retarded. You are not going against trained athletes whose whole world revolves around competition. If you do, chances are, you are going to seriously hurt each other, or simply wear your body down so when you reach 50 your ankles are swollen, your back is a wreck, you have a limp, and have had numerous concussions. That is not going to help you when you are a 70 year old cripple with a cane getting mugged by some young punk, you could have easily whooped on at twice his age.
I have knocked guys out with boxing training once or twice a week. When I was a brown belt, I took a 2nd degree black belt that said if he didn't want me to throw him I couldn't do it, and I slammed that guy so hard on his back he couldn't breathe. As a white belt in Judo I took a Judo teacher 40 pounds heavier than I, and while grappling I submitted him and he required chiropractic care for weeks. I did not train everyday. I did not do full contact boxing or fighting. I sustained very few injuries while toughening up and learning to defend myself. I am not particularly big or heavy, strong or fast, but by working smart I have learned valuable personal defense without tearing my body up. I tried competition Judo a couple times, and I realize now why some of these teachers can hardly walk in their 40's. I will not touch competition like that again because it is pointless for my future.