I'll admit to being an iconoclast...I strongly believe in the traditional styles. The more traditional, the better - I favor the Okinawan karate styles.
I have a dan (instructor) ranking in Okinawan Goju-ryu, with around 2 decades under my belt, so I am a bit biased that direction as well. I think that the hyper-traditional styles (by today's standards) are like piano lessons as a kid; maybe not where you end up, but a good start for learning the basics-in this case, the body's language and how to disrupt it!
That said, goju, while pretty strong in a stand up and fight sense, is not particularly long on edged weapons or floor work, so I have done a *little* bit of cross-training in judo, aikido, and (very) tactically oriented philipino and korean knife seminar work with some "names".
So far, I've mostly learned/been alarmed at how much I don't know, but at least I am familiar with some new and fun ways that I could possibly be attacked; forewarned is to some extent forearmed...
That said, if I was completely new to martial arts and just looking to quickly augment physical fitness training with down-and-dirty SD, I'd look first at basic defensive tactics training. I'd start by finding out the name of the defensive tactics instructor for the local PD (who will be way more on top of this stuff than would be required for the force in general; typically a "lifestyle" guy) and I'd work hard to make friends with them-DT guys need local training partners too. Second to that might be a commercial krav maga school or something.
Keep in mind, this is coming from someone who has earned some good muscle memory for dealing with a lot of the ways that one can attack and defend bare-handed over the years, so I tend to get a lot out of the "tactical" classes, by virtue of background; some things I am already very good at (by no means all though!), so I can build from there. I'd hope, however, that the defensive tactics classes would also be of tremendous use to folks with less training, since that's the whole point.
One easy way to get a good academic idea of what is out there for rapid acquisition of skills if you've mainly been a gun guy would be to pick up "Jim Grover's" combatives series from Paladin; that will at least give an overview of the type of things that one might look for when shopping for some training.
Just one opinion from a "hobbyist" type martial arts guy, take it with a grain.