Some characteristics of a street effective system are:
-simple & direct movements
-economy of motion
-functionality in all four basic ranges of fighting--kicking, boxing, trapping, grappling.
-emphasis on principles rather than techniques
My recommendation would be the Filipino Martial Arts/FMA. Kali is effective in all four ranges, and is the best defense against armed attackers, hands down. Its boxing curriculum (panantukan/pangamut) emphasizes limb destructions, elbows, finger jabs, and tight/compact punches. The kicks in kali (pananjakman) are all low-line; kicking above the belt in a street fight is too slow and too risky. The grappling aspect of the system (dumog) emphasizes quick and destructive takedowns, so as not to spend too much time on the ground. The art, overall, is multi-man attack oriented and has very sophisticated footwork based on the triangle. In kali, you learn the weapon first (nothing superfluous like bo staves; we're talking pocket knives, batons--things you will encounter on the street) and apply the weapons-based principles to the empty hand. Advanced practitioners become extremely proficient at improvised weaponry once they've internalized the attributes of weapons; they train with sticks and knives, but can then apply their skills to anything from a ballpoint pen to a key to a rolled up magazine.
Here's a video of some typical kali training: http://youtu.be/BtjAdhKlgTs (excuse the horrible music)
Jeet Kune Do (JKD) is also ideal for street defense. It blends arts like boxing, wing chun, brazilian jujitsu, fencing, FMA,judo, savate, etc. Its emphasis is on simplicity and adaptability.
Keysi Fighting Method is also effective because it is based on natural physical responses.
Now, to respond to some of the previous comments, while Muay Thai is a great training method for physical conditioning, it actually instills bad habits for the street. Muay Thai fighters are trained to eat punches and then to counter. While that's great for the ring, it's a terrible habit for an environment in which attackers are more likely than not to possess knives and other weapons. If you're used to taking punches to the chest, that same natural reaction will occur when you're attacked with a knife. Muay Thai can work for street defense only if its in tandem with some other art that's more geared toward self-defense.
Similarly, Brazilian jujitsu is EXCELLENT, but needs to be accompanied by other systems to truly be effective. Against multiple attackers, 95% of its techniques are useless if not dangerous. Try putting an attacker in an armbar while his friends are stomping your face into the pavement.
