Simplest and most effective... boxing.
You will learn more in 6 weeks boxing training than any other martial art.
You will learn more in 6 months of boxing training that you can use and apply than any other martial art.
After a year in boxing, you could even actually be half way decent. Not great... but you would absolutely use and abuse anyone who doesn't train regularly or fight often.
You can't say that about any other martial art... bjj, judo, muay thai, karate, etc... all have a higher learning curve than boxing.
And, no matter how long you study different martial arts styles... in the end everybody comes back to boxing because for stand up it is the simplest and most directly effective fighting style there is.
Lots of people will chant the mantra "all fights go to the ground." I haven't had anyone take me to the ground yet in a street fight... and the guys at my club who have done it have a black belt in bjj or judo. Everybody who fights today trains stand up and ground... and we always tell kids to train stand up first. It's much more valuable to get striking down before you even think of trying to get a ground game. It's like learning to ride a bike before you get a car.