Unfortunately there is no such measurement. Although I don't think it's impossible, one would have to take the following things into consideration:

Martial arts are usually created for a certain purpose in a specific context. Thus, to compare two arts you'd have to compare them in a specific context, otherwise it makes no sense. One of these special contexts ist MMA with its specific ruleset and other situational components, like a certain number of rounds, an octagon and so on. If you want to know which art fares best under these circumstances, you can test them against each other.

Does that mean these arts are also the best in another context, for example military missions, street-fighting, fighting against multiple adversaries? Likely not. While grappling is great against an adversary who is not versed in grappling himself in a one-to-one confrontation, it may be a bad idea to roll on the ground with someone who has a knife or his friends standing by.

It's also important to note that each rule in a fight profoundly influences the way techniques are done and how one protects itself. If head butts are encouraged, like in Lethwei (burmese bareknuckle boxing), fighters adopt different stances than say in Muay Thai where these techniques are forbidden. The same applies to attacks to the groin: if they are forbidden by the rules, there is no more need to protect oneself against them. This gives the fighter new opportunities in a competition but may prove a problem in a fight with no such rules.

One could argue that death matches as they have been common in Asia and also in Europe some 100+ years ago are way more "realistic" than MMA and that's probably true, but even these are situations where you need different skills than in the midst of the chaos of a battlefield or fighting in a crowd.

Many martial arts have evolved (or devolved, depending on the point of view) from a self-defense centered-training to a competition-centered training. Some call this "watered-down", but in fact it's the adaptation to another environment. You lose some self-defense-skills and you gain skills in the specific circumstances of a competition.

Also, many arts have lost their edge because they didn't have to stand any real tests or challenges for a long time. They just "believed" in their deadliness and didn't bother come out of their comfort zone anymore. Some have lost their inner secrets, some have just neglected the practical side of training: sparring. No wonder their proponents fail miserably when challenged by modern MMA fighters.

Nonetheless I think it would be great to define certain circumstances (such as a knife attack in the street, fighting in narrow space or in the dark, fighting against multiple armed or unarmed attackers, etc...) and evaluate what different arts have to offer and which one fares best.

Unfortunately there is no such measurement. Although I don't think it's impossible, one would have to take the following things into consideration:

Martial arts are usually created for a certain purpose in a specific context. Thus, to compare two arts you'd have to compare them in a specific context, otherwise it makes no sense. One of these special contexts is MMA with its specific ruleset and other situational components, like a certain number of rounds, an octagon and so on. If you want to know which art fares best under these circumstances, you can test them against each other.

Does that mean these arts are also the best in another context, for example military missions, street-fighting, fighting against multiple adversaries? Likely not. While grappling is great against an adversary who is not versed in grappling himself in a one-to-one confrontation, it may be a bad idea to roll on the ground with someone who has a knife or his friends standing by.

It's also important to note that each rule in a fight profoundly influences the way techniques are done and how one protects itself. If head butts are encouraged, like in Lethwei (Burmese bareknuckle boxing), fighters adopt different stances than, say, in Muay Thai where these techniques are forbidden. The same applies to attacks to the groin: if they are forbidden by the rules, there is no more need to protect oneself against them. This gives the fighter new opportunities in a competition but may prove a problem in a fight with no such rules.

One could argue that death matches as they have been common in Asia and also in Europe some 100+ years ago are way more "realistic" than MMA and that's probably true, but even these are situations where you need different skills than in the midst of the chaos of a battlefield or fighting in a crowd.

Many martial arts have evolved (or devolved, depending on the point of view) from a self-defense centered-training to a competition-centered training. Some call this "watered-down", but in fact it's the adaptation to another environment. You lose some self-defense-skills and you gain skills in the specific circumstances of a competition.

Also, many arts have lost their edge because they didn't have to stand any real tests or challenges for a long time. They just "believed" in their deadliness and didn't bother come out of their comfort zone anymore. Some have lost their inner secrets, some have just neglected the practical side of training: sparring. No wonder their proponents fail miserably when challenged by modern MMA fighters.

Nonetheless I think it would be great to define certain circumstances (such as a knife attack in the street, fighting in narrow space or in the dark, fighting against multiple armed or unarmed attackers, etc...) and evaluate what different arts have to offer and which one fares best.