Truly a loaded question. Everyone will have their own opinion, so here's mine. If you are looking for a quick, overnight fix, boxing is an effective martial art you can learn in a reasonable amount of time. Yes, there is footwork and movement involved, but at it's base it's really just learning about 6 punches. Make no mistake, it takes alot of time to truly master the sport though. Lately, more people are doing Muay Thai, which is also an effective martial art, but remember you need to be pretty flexible to be good at it. For some, flexibility takes a looooooong time to achieve. Others are naturally flexible. If you're one of those naturally flexible people (lucky bastard) then I'd recommend Muay Thai, since it's also a martial sport that's quick to learn the basics.
If you're willing to take time to learn a martial art, then I'd recommend pretty much any of them. Just try and find quality instruction. That's really the hardest part. I don't know how many masters of Tae Kwon Do, Okinawan Karate, or Judo there are, but every strip mall in the country seems to have a place to train. I believe pretty much all martial arts can be trained to be effective for self defense, some take much longer to learn than others. Aikido & Tai Chi might take 15 years before you're comfortably able to use them in a fight. Just remember, no martial art makes you bulletproof no matter what the instructor tells you. You'd be amazed how many claim to be utilized by the Navy Seals, etc. Don't let some guy try to sell you that you'll be able to fight like Jason Bourne. A popular fad these days is claiming that their art is the only art worth studying because % of fights end up on the ground. These statistics are true, but these are taken from police reports where officers are trying to handcuff a suspect and need to wrestle him to the ground in order to get the restraints on. Another gimmick today is that the instructor might tell you that their art is the most effective streetfighting style and have the documented proof to back it up. On closer inspection you'll find that the data is true, but pertains to 1960s street fights in Hong Kong. The lesson here is every martial art will say that theirs is the best...they have to, it's the way they recruit students and keep themselves in business.
Have realistic expectations of the art you choose to study. Movies are just that, movies. Fights don't happen the choreographed manner that you see them in. Rarely will others not get involved and let the fight go one-on-one. Using objects/weapons in a fight can happen. It doesn't matter if you train as a standup fighter or a ground fighter, or both: multiple attackers & weapons are going to make things very difficult for you. It's pretty easy for a troublemaker to conceal a handgun.
My last piece of advice (or others may say: more bullshit from this guy) is to train in something you enjoy doing. You have to put in the time and energy. When the instructor is teaching, pay attention. Watch for details. Listen, listen, listen to the instructor and to senior students. If you train in Krav Maga and you end up hating it, quit. You're probably half-assing through the lesson when you aren't busy looking up at the clock. The first martial art you choose might not be what you're looking for...or it just might be. Good luck.