Purely functional (meaning: bringing an unskilled person 'up to speed' as quickly as possible, en masse), mostly Krav Maga and "applied" Sambo styles (as mentioned above), or other military systems like the Modern Army Combatives Program.
For 'systems' that don't have 'minimize the time to learn it' as a goal, there isn't one style as such any more, imo. This is the beauty - no holds barred MMA competitions let the superior techniques float to the top, and everybody else learns from it and includes it into their training regimens. There isn't one 'style' that is taught, it's only people who are tested on their skills as a whole. And if one guy thinks he holds the truth in what style or techniques are superior, great - step into the ring, and put up or shut up. No more guild-like secretiveness, no more mystical uberman techniques or meditating for 15 years on a mountain top, no more 'I can't show you because I'd kill you'. In MMA, rationality prevails, once again.
I think it's great that people are sparring between disciplines and integrating various styles--but at the same time, this system can only act on its inputs, which are MMA fighters. I do wonder whether the culture of MMA prevents some talented martial artists from participating. I'm somewhat inclined to consider MMA more of a sport than a martial art--but perhaps this is just tradition talking.
The idea of an "MMA fighter" is a very modern notion. MMA started as a contest between practitioners of different martial arts, but it quickly became apparent that Brazilian Jiu Jitsu trumps any other style. The first UFCs were effectively a debunking of almost all martial arts. MMA fighters all derive their style from the Gracie family because that was the only way to beat them - combine Gracie techniques with stand-up skills derived from Thai and western boxing.
Either something works or it doesn't. Tradition doesn't, precisely because it's tradition - a rapidly evolving style will easily beat one stuck in the fourteenth century. The Gracies dominated early MMA because they had decades of experience in Vale Tudo matches, fighting all comers in bouts with few or no rules. They knew what worked and what didn't through bloody, painful experience.
Martial arts in the traditional sense are simply woo, no better than homeopathy or divining. They have been tested and they don't work. There might be some value to what is essentially a form of dancing dressed up as combat, but take it into a ring, a cage or a pub brawl and the best you can hope for is that it is useless - most traditional arts are actively harmful.
Yes, I agree, MMA is more a sport, but as I mentioned in other posts in the discussion on what constitutes an 'art', what are considered 'martial arts' don't really have the 'fighting' aspect as a central goal any more. Which is OK, I'm not making a moral judgement against traditional martial arts, I'm merely classifying.
And yes, some of the rules of MMA make it so that some techniques (groin shots, hair pulling, nipple twisters) cannot be used. But if the underlying suggestion is that there are somewhere out there in the jungle people who do use these techniques but they just have to suffer their superiority in solitude because their lethal pinky fingers prevent them from having interactions with mere mortals, I strongly disagree. My point throughout this discussion is (and I guess it's way off topic for this site...) that 'martial arts' have had their time in the context of systems who claim to contain or teach effective fighting. They're OK as systems of ritualized techniques to be practiced recreationally, but have been proven as in the best cases inefficient and in the worst cases wrong when it comes to teaching practical skills.
The early UFC fights (for all their faults) did show conclusively (at least, I don't really see how to explain it differently, nor have I ever heard anyone doing so) that one-sided training and a reliance on stylized movement are inferior, even when deployed by top practitioners, against techniques that were trained and developed in circumstances that mimic 'real' fights more closely. And I do understand the apprehension the more traditionally schooled feel against the new breed of fighting styles; but that's just another manifestation of man's natural tendency towards conservatism, I guess.
That sounds nice. Martial arts always had this real woo woo feel to them. If I want to learn to fight, I want to learn to fight, not to meditate. (Perhaps that's simply a brash Western attitude, though).
For 'systems' that don't have 'minimize the time to learn it' as a goal, there isn't one style as such any more, imo. This is the beauty - no holds barred MMA competitions let the superior techniques float to the top, and everybody else learns from it and includes it into their training regimens. There isn't one 'style' that is taught, it's only people who are tested on their skills as a whole. And if one guy thinks he holds the truth in what style or techniques are superior, great - step into the ring, and put up or shut up. No more guild-like secretiveness, no more mystical uberman techniques or meditating for 15 years on a mountain top, no more 'I can't show you because I'd kill you'. In MMA, rationality prevails, once again.