the most effective martial art for a streetfight.
Would probably include a lot of dirty fighting. KM is big on crotch kicks. I am training mma to cage fight, and as amateurs, we actually do have a few rules - no elbows, no knees to a downed opponent, no eye gouging, biting, or crotch blows. These can also be read as being effective street tactics, so effective that they are against cage fighting rules.
If you are in a street fight, that should mean that the other guy(s) started it, and you did everything possible to avoid the fight. In that case, there is no such thing as dirty fighting. Look for improvised weapons first, almost anything can be a weapon, throw dirt in eyes, rocks at head, (when I was a kid, I always had a good arm, and rocks were a great equalizer against playground bullies. I can remember being about seven years old and drawing a lot of blood from kids much bigger than me). Any weight like a rock or chunk of metal can be wrapped in a shirt or towel and swung with tremendous force. Put a pool ball in a tube sock and swing that around some time. It's lethal. Beyond weapons, you then strike to the crotch, eyes, and throat. There's not much glory in winning this way, but you do live to fight another day.