No real training. Come to think of it I haven't even been in a fight since 2nd grade (and those fights were considered violent if slaps were thrown around). Basically my form of self defence is looking somewhat like a 250-pound half deranged hobo.
But I do have some knowledge of popular fighting stiles and favor Krav Maga. Here's why:
Krav Maga isn't what you'd actually call "martial arts". Martial arts are fairly ritualized (especially japanese/chinese ones) and focus on form, technique and are somewhat adapted for competitions, making them safer for both involved. Krav Maga on the other hand is all about survival. Generally, there are no rules in Krav Maga, as it is a defense fighting technique which is not regulated, but utilized to keep the user safe and incapacitate the opponent by any means necessary. There is nothing holy (half the moves I've seen involve a hard part of your body and your opponent's balls, usually in comedic and eye-watering combinations). It's also considered the easiest and most efficient form of self defence since anyone, regardless of age, sex, prior training or physical condition can learn it very fast. It relies on short, efficient movement (none of that fancy jumping around, just a plain old elbow to the face and knee to the groin, thank you very much) and details many ways to remove and defend yourself against many common weapon threats(knives, guns, blunt weapons, etc)
Boxing is probably the most useful background to have if you want to pick up Krav Maga since it emphasizes punches as basic strikes which are useful in almost any situation
As a form of self defence I really consider Krav Maga to be the best choice out there. Other martial arts have, of course, other things in their favor: discipline, physical training, philosophy, but for pure self defence, you can't beat Krav Maga.