Short answer: MMA.
Long answer follows...
What we've learned about martial arts in the past two decades is that what matters most is how you train, not what you train. Style and techniques are unimportant. So long as your training involves sparring against "live" and "non-compliant" partners, the techniques you use and how you use them will be honed to fit that situation. And that situation, by the way, most closely approximates a real fight.
What does it mean to have a "live" opponent? It means that your partner isn't a robot. He's not going to tell you what he's going to do. He gets to decide that. And he doesn't stop once he completes what he was doing. He keeps going, reacting to what you're doing. He's alive.
What does it mean to have a "non-compliant" partner? It means that your partner is actively resisting everything you do. He's not letting you do stuff to him. He's trying to win against you. He is doing what he can to stop you from doing whatever you're trying to do.
So long as you have live and non-compliant partners, you have the basic recipe for success. Your techniques may be limited and ineffective to begin with. But over time training the right way, you will learn what works and what doesn't. And more importantly, if you have a hole in your game, you'll go out on your own to figure out how to fill that hole.
For example, if in sparring your partner is repeatedly able to shoot in and take you down right as you're beginning to punch at his head, you'll learn to adjust your stance so that you're not quite as open for that shoot. And you'll learn how to sprawl from that stance.
The techniques follow directly from that practice as a matter of necessity. So style doesn't really matter. What matters is how you train.
As for your question about how to "measure" the effectiveness of a martial art, we have a method, though it might not be what you're really asking for. It's called MMA.
In MMA, people from any martial arts background can spar or compete against people from other backgrounds. From these matches and from MMA training, we can see trends. We can see which styles are, on average, producing better MMA fighters.
The styles that, on average, seem to produce the most winning MMA fighters are: Brazilian Jiu Jitsu, Thai Boxing, Wrestling, Boxing, Sambo, and Judo.
At least that was true in the early days of MMA fighting competitions. Back then, in the early 1990's, people weren't training in multiple martial arts. Most were "pure" stylists. And those that did the styles I just listed tended to do better, on average, than all the other styles. It's because of the fact that these styles train people using live, non-compliant partners who are actively trying to win against them.
Later on, people realized they needed to train in those styles. So they all went out and learned stuff like Brazilian Jiu Jitsu, Muay Thai, Wrestling, Shootfighting, Pankration, Sambo, and so on. And that became MMA.
Now here's the interesting part. There have been some noteworthy MMA competitors who had training in martial arts that don't make that list, however. And they did extremely well. So what does that tell you? For example, Lyoto Machida and his Shotokan Karate background.
The thing to keep in mind is that, once again, it's not the style but how you train that matters. Lyoto Machida wasn't just a karate practitioner. He also has a black belt in Brazilian Jiu Jitsu. And he practiced MMA fighting. But he was able to bring some of his karate techniques and strategy into his MMA game. The result wasn't a combination of karate and MMA. It was just MMA.
MMA training is like a crucible. It burns away the ineffective elements. What remains just works.
So then your question about how to measure the practicality of a martial art is answered: Find out how people from that martial art do, on average, against others in MMA style fighting competitions.
This has to be relative, not absolute. And you're not comparing the martial arts against each other. You're actually comparing the people who have trained in those martial arts. And you can use averages or take the best. That's up to you.
Of course, this is really hard to do nowadays. It's because in MMA, the level of skill and knowledge has increased tremendously. Nobody goes into MMA competitions these days without first learning some Brazilian Jiu Jitsu and Muay Thai at least. So when you're trying to see how a martial art does against other styles, it's really hard, because you can't find pure stylists in MMA these days. And there's a reason for that.
Now, as for weapons, there are some MMA groups that add weapons to their sparring. For example, Dog Brothers. They also do MMA without weapons, but they'll tell you that they had to modify what they do to deal with knives and sticks more effectively.
Sorry if you're looking for something more analytical and objective. If you want some kind of a number representing the practicality of each martial art, you're really not going to find anything useful.
You could count how many techniques a martial art has, but as I said above, it's not the number of techniques that matters. It's how you train them. Most techniques are ineffective anyway and will just be thrown out when you go to MMA.
Or maybe you could look at physical qualities like mechanical advantage, leverage, speed, and power. That would just measure the effectiveness of each technique in isolation. It wouldn't tell you how useful or how practical those techniques would actually be.
You could argue that the more useful techniques are indeed the most direct, fastest, most powerful, most leverage providing techniques. Still, it's not obvious that the one must follow from the other.
And consider techniques like the back flip. You can make a perfect back-flip that's really fast and mechanically efficient. But is it practical to use in a fight? Probably not.
So I'm back with one answer: It's how you train that matters.
It's very simple. You could take two martial arts styles that each have exactly the same techniques in them. But maybe one style emphasizes body conditioning and spends most of the class doing strength training, cardio, and so on. And the other one instead uses the time to do martial arts training with a live, non-compliant partner. Which one will produce better fighters, on average? My feeling is the second group would come out ahead.
It's how you train that matters. Not the style. Not the techniques of the style. Not how many techniques you have. Not how strong or fast those techniques are.
Hope that helps.