[AMF]

My two cents, given my limited experience:

Boxing if you want the quickest return on investment of time - you can learn to take someone down fast, and learn how to do it quicker than in other disciplines. Good for sucker punches or straight up duking it out in a street situation, but hard for the long fight or if you need to do anything on the ground or other than punch and dodge punches.

Judo is good for learning how to street fight as well. Most street fights go to ground very quickly, and so Judo is one of the better for learning how to grapple, which is what you generally spend most street fights doing.

Karate is good if you have a long time to study and get good at it. (couple years minimum, IMHO). But only at the higher levels will it be anything other than counter-productive in a street situation.

Aikido is excellent for smaller builds (and women tend to be very good at this too, IIRC) and for learning how to use opponents strength against them. Also good for incapaciting joint holds. However, takes many years to get to the point where you can do it effortlessly. Some of the most amazing things I have ever seen have been done by high level Aikido masters.

I know nothing about any other disciplines, so I am probably missing a bunch of good things here.

Of course, the standard caveat is that if you're looking to learn a martial art to kick butt, you probably shouldn't be doing it in the first place. However, at higher levels, most martial arts teach the confidence and control that enables one to avoid fights to begin with. Nothing more disarming than someone who projects that confidence of "I could kick your *ss, but I choose not to, since it is wrong, etc..."

Again, only my two cents.
Alarik