My take on self-defense...
With respect to all who have posted, here's how I looked at it when I started looking at self-defense.
All fights start stand-up with grappling or striking. Some end right there while others will move to stand-up grappling if not there already. Lastly some end on the ground. Stand-up grappling includes hands on pushing/ shoving, charging for a takedown, holds/chokes, and throws/takedowns. Stand-up striking is fists, elbows, knees, and feet.
If in a bar, parking lot, or other non-controlled environment where there are other people never willingly go to the ground with your opponent, even if you know what you are doing. You don't know which people are his friends. There is no defense against a stomp to the head from another person while you're fighting his friend on the ground that I know of.
Your fighting system
1. ...must be stand-up focused. All fights start there and many end there.
2. ...must completely stop an opponent's attack within five seconds of the fight starting and subject you to as little damage as possible.
3. ...must be 100% actively practiced in class against a fully resistive opponent. Learning moves against a moving opponent who is 100% fighting you is a completely different ballgame.
4. ...must include some groundwork. Groundwork must be focused on moves which end fights immediately (chokes/joint locks/some of the painful pins). Wrestling, though a fantastic workout and great grappling art which can cause pain, is not a self-defense art.
5. ...should use hand/elbow strikes to vital points (eyes, throat) or low kicks to knees and groin. Straight fist punches to the face will not be emphasized due to broken knuckle/hand. High kicks will not be emphasized.
I am biased against most traditional striking systems of kung-fu, karate, tae kwon do. I have not seen realistic fighting on a regular basis in their classes.
I am also biased against striking-only arts in general. While many fights start striking they almost all go to stand-up grappling and some go to ground. Stances and balance the average joe and trained strikers use go out the door against the stand-up grappling phase. If I had to look at a system focused on striking I would look at kickboxing, muay thai, krav maga, jeet kune do etc.
I am familiar with
Jiu-Jitsu - (japanese) The only complete martial art from stand up to groundwork. It includes striking, all joint breaks (finger/wrist/elbow/ankle/knee), all chokes, throws, and painful pins.
Pro: Fights can be stopped from a basic finger/wrist lock standing up or go through a full combo of strike/joint lock, throw to ground, choke/lock groundwork. Completely and utterly devastating on every level.
Con: Some are more kata practice focused then active sparring focused. Also there are a lot of moves involved here. It will take a little longer before you are proficient. I would be very picky about choosing your place.
Judo - Developed from Jiu-Jitsu. Throws, chokes, elbow breaks, and pins are taught.
Pro: Like Jiu-Jitsu, this is the name of the game for close quarter hands-on fighting. A throw against a hard floor or concrete is a fight ender. Active free sparring is emphasized. Groundwork is taught 10% to 50% of class. It will be adequate to take care of most opponents.
Con: Though strikes may be taught as setups to throws there is no distance striking taught. Secondly, stand-up grappling takes longer to learn than either striking or ground grappling. It will be six months before you are proficient against most opponents.
Brazilian Jiu-Jitsu - Developed from Judo.
Pro: The name of the game in groundwork. It is Judo's, which is by virtue Jiu-Jitsu's, ground game. However, it has expanded beyond that. It is taught better than either currently does and has clearly expanded beyond Judo/Jiu-Jitsu's original moves and mindset. Almost all the time is spent groundwork. Additionally, groundwork is also learned much quicker than stand-up grappling. Three months and you will be taking the average joe on the ground. It's focus on active sparring is superb.
Con: Most clubs spend less than 5% on stand-up. This makes it a great compliment to another art but is not a stand alone self-defense system.
Myself
I chose Judo three years ago as it covers the stand-up grappling phase of fighting and will cover most guys on the ground. I may look into a striking system in a couple years.
Even with my bias against striking, anything you choose and spend time on, will put you a notch above the average guy. Your physical stamina may increase, you may not panic as much when someone is in your personal space, and you will have a lot of fun.
If all you are looking for simply a great workout then I would choose BJJ, wrestling, judo, jiu-jitsu, or kickboxing/muay thai in that order.
Good luck!
