I'm gonna go with recommending Krav Maga too(although it's not a Martial Art,it's self defense),I started Martial Arts 30 odd years ago with Kung Fu,and have done just about every interesting one to come to light since then(Shotokan Karate,Bujinkan Ninjutsu,Muai Thai(in Thailand),Pencak Silat,Tae Kwon Do...I'm sure I'm forgetting a few!..
and if you don't like Krav Maga a couple of other good picks are Pencak Silat(the Martial art used in the Raid Redemption and Raid 2 movies,Muai Thai fighters are some of the toughest people I've ever met because of the ferocious training schedule,there's also Bokator which is Cambodian,but if you make the mistake of mentioning/asking the question "which came first Muai Thai or Bokator you may not survive the answer!)
If it's fitness that you want by all means do some MMA training,but remember it's a sport...with rules,and I heavily discourage anybody who thinks taking a fight to the ground as soon as possible is a good idea...one of my friends was murdered in Wales a few years ago because he took a fight to the ground and got kicked to death by the guy he was tying in knot's friends and he was a very competent fighter,taught me a lot of Jiu Jitsu moves that I still use today.
Ultimately I would recommend a Krav Maga school whose main instructor has been trained by someone from Roy Elganyahan's school
https://www.youtube.com/watch?v=5hXjr_7bSdg,anyone who has trained with Amir Peretz or of course the main man Eyal Yanilov,
I know this is a wall of text but the most controversial aspect of Krav Maga training is Gun Disarms,I was totally against them when I started(as I shoot as well,and if I'm gonna shoot someone it's not going to be within reach of their arms),
but those disarms are mainly for execution style situations where someone is intending to shoot you from close distance,so I got over my reservations
I would also recommend Escrima,and a good Krav Maga or Escrima class will get you every bit as fit as Boxing or MMA,
One final word,street fighters with no training,who fight all the time,beat black belts and highly trained individuals all the time,there's a world of difference between a Dojo or Ring and a street or bar situation,so if you're REALLY serious about learning to protect yourself,and read your surroundings,either get a job as a Door Supervisor(we used to just call ourselves Bouncers,but a lot has changed since then!) for a while,or get ADVICE from a Doorman/Woman who has at least ten years on the job,
also look up a chap named Geoff Thompson http://www.geoffthompson.com/ and read his book "Watch my Back" it's full of tips and tricks that have saved my ass numerous times and is a cracking read too.