Well, I don’t have the experience in it that most of you have, but I’ve noticed some arts that imho work better than others, and have some opinions that are probably mostly speculation, but I’ll share them anyway. 
I’ve taken a few styles of Karate and lightly sparred with friends who have taken anything from Tae Kwon Do to Hapkido.
Some of the styles of karate were about style, the moves were shown over and over again and we just learned to parrot them, kind of like dance class with punches. This is partly the style and partly the instructor. The styles often focused on belts, as in, learn these moves to get a belt, not learn the moves that help in your situation. Especially useless were the moves like the high kicks, the complex stuff. I’ve never seen it done except by the best (in technical skill) martial artist I know, and only then against an unskilled partner.
To contrast this, I took Uechi-Ryu (sp?) for a while and found it incredibly effective. It probably also helped a bit though that I had an instructor who sounded a bit like Glitch. This style focused on short fast movements, the kicks that they used were more like stomps, and used from that sort of range. Even the knee shots were done at arms length, unlike the more distant styles I had been used to. The philosophy of the style was good too, everything that could hurt the opponent was used. Why block when you can smash the opponents arm, then why let them take it back when you can grab and break, etc. The basic circle block extends perfectly into a smash, wrap, and hold while the other arm hyper extends their elbow.
The short fast style fits much more closely with the ‘street’ fighting I’ve done.
I’ve never been in a fight where I’ve been able to control the distance (keeping it distant). The enemy can always close faster than I can back away. A style that accepts this and is designed to function in close works a lot better for the positions I found myself in.
As for grappling… Well, against two opponents, you’re toast if it ends up there. You’ll be applying a choke hold while bad guy #2 takes a leisurely kick at your head. It’s important to know enough about grappling that it doesn’t take you by suprise, but for most people’s purposes, by the time it gets to grappling, you’ve already lost.
I can see the situation being different for a policeman or bouncer, who are expected to pacify, not break, the opponent. But I’ve only ever been in one fight where I had any intention of going easy. (When I pissed off a friend who was drunk and had just broken up, he was mad in general, not at me. Breaking him wouldn’t have helped either of us.)
I think that the most important tool is attitude, not only being willing to hurt the opponent, but being ready and not freezing up. Just a basic no-skill punch to the face will hit most people (I doubt more than 1 in 20 could block a punch they didn’t know was coming) and would be more effective than a complex but never used technique you didn’t use to the full extent. As Glitch and others said, sparring and SBTing are the only way to achieve the readiness for action.
An attitude of wanting to subdue someone who attacks you in laughable, imho. If someone wants to hurt you, they’ve just handed you a signed form saying they renounce all rights to civilized treatment. Break them until they stop moving. If they’re still alive, well, lucky them, but it’s not really your concern unless you’re highly trained enough that you should be expected to know how to subdue without hurting. (Like a bouncer, cop, etc.)
The arts I’ve found effective when used against me and the close-range style I’ve ended up with (and been attacked with most often) are Uechi-Ryu karate and Hapkido. Both are very violent arts, with little concern for pacifying an opponent. Uechi-Ryu concentrates on beating someone very hard and very fast, not as much in sensitive places as everywhere, with a preference towards especially breakable bits. Hapkido is, to my eyes, Akido for bad-asses. It’s the same sort of thing my mom takes (Akido) but without the focus on letting the opponent walk away (or ever again for that matter). It extends the holds a bit farther till they become breaks, and it teaches doing this on the fly, not landing on and holding the opponent.
One thing both of these arts have in common is that they both de-emphasize style and stance. There is a stance in Uechi-Ryu, but it’s a basic forward facing standing position with the legs bent a bit to drop the center of gravity and the arms up, ready to attack or defend. None of this lame horse stance or anything.
My friend who had taken Hapkido was a scary person, I’ve never had more of a feeling of “he could utterly destroy me if he wanted to” than from this guy. Not because his attitude was such, but because everything he had trained to do ended in a joint or bone being rendered useless.
For defense, take a basic defense course and carry pepper spray and that key-wand that people mentioned. And use them. Chances are that you’ll never meet anyone who knows enough of a martial art to matter, people who can invest years in training rarely have to mug people for drug money.
If really feel the need for defense against trained people, take a quick, close, and brutal art.
This is a point that could end up in the GD, but I’ve never met anyone who I’ve considered a good fighter who had worked with the ‘Ki’ side of the arts. I don’t believe it helps at all except in the discipline area, and only if you feel you want that. It won’t make you more effective of a fighter.