Like all martial arts it depends on where and how you train. If it is a good school with a good instructor it is a great martial art, again I say that like any other martial art/martial sport, it's effectiveness is based on how seriously you train and absorb the knowledge passed on to you.
The truth is there is no best martial art for self-defense. All and I mean all martial arts have holes in them. The best thing you could do is study as many styles as you can. Striking, ground fighting, and joint control are the big three concerns to consider. I'm not sure what you're planning on training for but some martial arts to check out are Aikido, Kung fu, Judo, Ju Jitsu, and Military combative school (if you can find one). Studying multiple martial arts is the only way to cover all you bases, finally choose what it is suitable to you and help you in your battle. I hope this helps.

Why Traditional Karate Is Not Effective for Self-Defense
Is Karate being a good self defense a myth ?
Is karate effective for self defense or just a sport?
The Right answers about Effectiveness of Karate in Self-Defence should be: "If you're up against someone who doesn't know how to fight -- yes, old-style karate can work, but if you fight an experienced street fighter or a trained fighter, no way!" - Jon Bluming 
Firstly :What do you *mean* by the term "self-defense"? What sort of situations do you envisage that require some sort of "self-defense"? Single or multiple opponents? Armed or unarmed? Size relative to you? Do you expect to be grabbed, thrown, kicked, or punched? Can you speculate on the motivations for an attack? Do you expect merely to be robbed, or do you consider rape, maiming, or murder a possibility? These are very unpleasant questions to think about, but they're necessary to figure out what your *personal* definition of "self-defense" is.
 
By WR Mann
The underlining motivation in studying any type of martial activity is to protect ourselves (or others) in a real fighting situation. At first glance karate seems to provide a solution, until you look more closely at its underpinnings; then you realize it's not equipped to handle violence in the 21st Century. I often refer to karate (and other traditional Asian martial arts) as the Potemkim Village of the martial arts -- a grand facade offering significantly less in the way of substantive tactics and defensive measures than any of the reality-based defense systems.

Recently, while speaking to friends visiting from Australia, the topic of self-defense came up for their daughters (age 9 and 11). They mentioned there was a karate school in their neighborhood and were considering enrolling them there. That sent chills up my spine, and with the same fervor as a surgeon desperately trying to save the life of a stroke victim; I informed them that karate would produce the least beneficial results.

The reason I dissuade people from getting into karate (and other traditional martial arts) is because I don't want them misinformed like I was, studying retrograde theories and techniques that no longer have any relevance to the way we live and need to respond to. Let's be honest, all things being equal, some fighting styles are vastly superior to others. I'm not saying karate is completely ineffective (Bruce Lee did). Karate, like many other fighting styles, has the potential of stopping an attacker, however, the degree of efficiency is far less than muaythai, Brazilian jujitsu, boxing, and especially reality-based systems. Using a metaphor, the flintlock is certainly capable of stopping someone, but the M16 has a far greater degree of efficiency.

To properly put this question into perspective (why karate is not effective as a modern self-defense system) we must first discuss four topics:
1) Conditions of violence in the world today
2) Constituents of effective self-defense in the 21St-Century?
3) What are people looking for [in their self-defense training]?
4) A differentiation and clarification of fighting categories in 2003