Another vote here for KM. (Based on the OP's goals)
I've been training BJJ (with mma/MT) for about 4 years. I had always dismissed KM as another McDojo, "martial art of the week", but about a year ago, I started training at a CrossFit gym that teaches KM also, so I started taking a few classes here and there (initially just for the workout) and was really surprised! What they really ingrain is instant trained intensity. Meaning going from zero to 100% at the flick of a switch and just drilling turning on the rage and turning the tables on your attacker.
There aren't any new moves or anything, they just drill executing the basics from real-world fight situations. So instead of drilling a rolling omoplata counter to a single leg takedown, you'll drill headlock/choke/grab escapes and every way conceivable to hit someone hard in the nuts. They also don't stick to a "one-size-fits-all" approach. They'll say, like here's 2-3 ways to get out and you do each one a few times, and then for the rest of the drills, you pick the one that works the best for you and worst for your attacker. They'll be like, "escape and deliver ten blows." They don't care what they are as long as they're hard and to a vital soft target. The other thing they teach is situational awareness. Your goal, period, is to GTFO, so if someone is attacking you, do just what is needed to neutralize their attack and immediately check around for his buddies. If it looks clear, make sure he neutralized, then get away. They also make you think about being boxed in, aware of your exits, etc.
I really like the nature of the drills. Many of them you start with your eyes closed, or boxed in from 4 sides and you don't know how they will attack you or from what side and you have to fight it out. If you are too focused on the one attacker, they will happily come up and blind-side you with a pad to make sure you are watching out.
So I think for pure self defense, it's hard to beat! The only think I fault them for is maybe not spending quite enough time on the fundamentals. A lot of the folks in there come from martial arts backgrounds, so they do just fine, but knowing the basics would be a nice pre-req. (Like 6 mos of TKD/boxing/MT, etc.)
Again, this is based on the original posters goals of learning some basic self-defense. If you plan on being a cage fighter, it may not be for you. MMA guys aren't going to put you in a headlock, or try to "rape choke" you, so spending time there (and drilling hitting places you can't hit in sport MMA) is counter-productive.