that's not music you hear (that's the devil)

What martial art do you train? Brazilian jiujitsu, MMA, muay thai, judo.

What martial art should I train? Whatever you think looks cool. If you have none in mind, consider whether you'd prefer grappling, striking or weapons. Then ask yourself whether you'd like to beat up people who don't want to be beat up, and who are in turn trying to beat you up. If yes, look for a martial art that offers regular spars against resisting opponents. This includes BJJ, muay thai, MMA, (kick)boxing and judo, but there are many, many others.

What martial art is best for self-defense? I don't subscribe to the common wisdom framework of self-defense. If you want to beat up people who are in turn trying to beat you up, see above. On principle, I advise against krav maga and most other forms of military tough guy shit, especially those purporting to teach techniques too extreme for live sparring.

What's your miscellaneous opinion on this or that martial art? I can personally vouch for BJJ, muay thai, MMA, (kick)boxing and judo being extremely fun and conducive to beating people up. I have nothing but contempt for krav maga and most other forms of explicitly military martial arts. I try not to harbor opinions on martial arts that I haven't practiced. I am wholly uneducated on the matter of weapons.

I want to train martial arts but am unfit: Fitness is a moving target. You get fit for your martial art by doing your martial art. Any gym that filters you on account of your fitness level is not worth attending. That said, if you're coming from a long-term sedentary lifestyle, I recommend three to six months of weightlifting to condition your muscles enough to avoid injury. My own conditioning routine is kind of idiosyncratic; I recommend bothering any of the women on here who powerlift or bodybuild.

I want to train martial arts but am chronically ill: Without further details on your illness, all I can offer is that exercise is a must for any human, which doesn't mean you should destroy yourself pursuing it. As above, you may want to condition yourself aforehand, but any gym that filters you on account of illness is not worth attending. This is one case where I specifically recommend BJJ, as a low-impact art where pushing yourself is basically discretionary.

ngh., wjat if we fought.., aand then we kissed >.<: Please offer me a more interesting quarry than this.