I was thinking of doing some Karate

I have heard that kickboxing is a real aerobic workout, although I don't think that is considered a martial art.

I have also noticed that people who train ju-jitsu get a good workout as well.

Tai Chi Chuan plus weapons

Since I do karate, I think it is but if I didn't I'd think that all martial arts were the same. Karate, aikido, judo, tai kwon do, tai chi, and etc. all are similar but they specialize in one area, but I think for aerobic exercises, they'd all be around the sameness of aerobicness.

Kickboxing is definitely the best. The trick is deciding what type of kickboxing. The cardio kickboxing at a gym can be a decent workout.

I've done Muay Thai Kickboxing that focuses more on fighting techniques. Even if you aren't aiming to learn how to fight it can still be a workout. The place I go to to train spends half of the one hour class working on fast-paced conditioning drills so that you'll be strong enough for your punches/kicks to be effective. The other half works on refining the techniques with a lot of repetition. That repetition is also good because it can tire you out. I vote Muay Thai Kickboxing.

I recommend Judo because it is effective self-defense, a practical martial art, fun, respectable, safe and highly physical.

Also, Judo exercises the whole body.

The best ANYTHING for fitness is what you enjoy the most. You'll do it more often, you'll do it more intensely, and you'll really enjoy it more. Having fun while you technically exercise is the best thing.

I have tried several different martial arts. For fitness, training for boxing is the toughest. I would follow that up with wing tsun (kung fu). Both are constantly moving, constantly throwing strikes, and constant effort.

I would never try to apply a martial art in a real life situation. In a real life dangerous situation, get away and live. Don't try to karate or judo someone.

warmaker : I think he is asking our opinion, unless this is meant to be a strict debate about which martial art is the "best" for fitness, to which there is no single answer.

Comparing strategies in self-defense is going to derail this thread fast, but I would like to point out that Judo is a self-defense martial art that does not encourage fighting. It is very applicable in a real-life situation and there is no reason you would not use your martial arts knowledge (in Judo or Karate or otherwise) to achieve the end goal of self-preservation. I think you are implying that someone practicing martial arts such as judo or karate must be committed to unrealistic expectations of the effectiveness of their techniques. This is not the case. Neither Totts nor I mentioned "Judo-ing or Karate-ing" someone and I merely mentioned Judo's practical self-defense value as a bonus to it's fitness benefits as I felt it was useful in the context of this thread. I think it is unhelpful to begin a debate over whether martial arts are practically useful in a real-life situation.

Middlemoor : First, I never mentioned strategies or which one was better as an actual combat technique. I said training for boxing is really tough. Then I said my two selections involve a tremendous amount of movement. Ultimately, I said it's whatever the person enjoys as a sport and that enjoyment is what makes them work harder.

Second, it's unsafe to take classes for a year and try them in the real world. I thought it was appropriate to mention that. Sure, it wandered off topic but it is an important point to bring up to someone who now thinks he or she wants to learn karate and go fight bullies. Maybe you and Totts didn't take anything from it but someone else who's just reading may have.

I appreciate you keeping things on the topic but I didn't get too far off.

Training for boxing is still the toughest in my book. It all comes down to how much effort you want to put into it.

What I was referring to by "comparing strategies" was the specific mention of Judo directly after my post, which basically inferred to me that my mention of it's self-defense applications was ill-advised or false, as you flatly advised, "Don't try to...Judo someone." In your opinion it's unsafe to take classes for a year and then try them in the real world. I would argue that once you are forced into a real-life self-defense situation, any course of action will have an inherent risk. However, this is besides the point. Once again, I feel this is steering the conversation away from the original topic, because I never insinuated that anyone should "try out" their martial arts training. I merely said it was practical in a self-defense situation. I take that be one where you are the non-aggressor and it is appropriate and unavoidable for you to take some physical action to avoid harm. I am hardly suggesting that you chase down a mugger and attempt to put him in a key-lock. And I mentioned this only as a bonus to the fitness aspect, not as a promotion of martial arts for self-defense nor to incite a debate about it's effectiveness. I don't see any suggestion from Totts that his/her intention is to "go fight bullies". I think it's appropriate to mention the side-benefits of a martial art in the context of deciding which one is the best for fitness (such as practicality, difficulty, mental and spiritual benefits, etc) but inappropriate to begin debating each one's practical effectiveness.

I did Taekwondo, it's a good way to have fitness while learning self-defense. 

I'm currently a red belt, but I quit because I had to move to a different town =(

I'm not a ninja but ive actually done karate,ju-jitsu,kung-fu and taekwondo even though noone believes me apart form the people that i did it with but tbh i found actual fitness training is alot quicker and faster. to be fair i only got to white belt in all the martial arts apart from an orange belt karate but in my defense they were ridiculously hard especiually kung-fu. If your looking for the best for fitness though, definatley do kung-fu its all jumping and spinning and running about and the tutor guy could do the wall flip thing looked awesome :o os i definatley recommend kung-fu.

Karate chops

idk but muay thai and judo keeps me pretty fit and not to mention work

Kick boxing has to be the best. You get your whole entire body moving around and you get everything working at 100 miles an hour and you can really burn a lot of calories with the kick boxing routine.

Armizare
It's medieval Italian martial arts with sword, dagger, grappling and other techniques that have gotten me really in shape. Personal bias but I love it. Though I'm sure many others would work well. As stated by others, it depends on what you like and would have fun with.

I never really tried out, but apparently Tae Kown Do is a good martial art, since it uses arms and legs for fighting, and maybe Karate too, in my opinion all martial arts are good for fitness.

I can not answer that question honestly. I can, however, tell you what worked for me: when I was playing little league baseball, I did Tae Kwon Do, and it kept me in quite a decent shape for baseball. So, needless to say, I recommend Tae Kwon Do.