Let’s talk about “practical” for a second. In the world of martial arts, and really everything associated with combat, “practical” is a loaded term; it refers to any style or weapon that’s intended for actual combat. It’s distinct from sport or non-combat martial arts, like Tai Bo. In the case of weapons it distinguishes between actual combat weapons and display weapons, like the rainbow knife on my desk.
So, if you’re asking, what’s the most effective combat style, then, whatever fits. There are plenty of active combat forms available to civilians, and military or police characters will know their organization’s hand to hand form. It’s not uncommon for police to actively start looking into other martial arts as a result of their training. Similarly, as I recall (and I could be wrong about this), it’s fairly common for military personnel in overseas postings, to pick up local martial arts and bring them back.
Generally speaking, practical styles split into two families, with a lot of crossover; subdual and lethal. Subdual styles involve restraining the opponent, and holding them in place, usually via joint locks, throws and holds. Most police hand to hand forms, and almost all self defense training are focused on subdual.
Lethal styles are ones that involve quickly breaking someone so they stop screaming and thrashing. Almost all military styles fall into this header. Some exceptions are Chin Na and modern Systema, which borrow heavily from subdual techniques. Where most subdual forms are content to lock a joint, lethal styles will frequently follow with a break.
If your character is a civilian, then you’re probably looking at any of the modern self defense schools. It is probably the most prolific, practical martial style today, and easy to explain in a character’s back story.
If you’re looking for something slightly more obscure, then Krav Maga or Muay Thai are both options. But, Krav Maga is about a decade out of date from the actual military form, and Muay Thai is technically a sport form. Granted, that sport involves tagging someone in the kidneys until they piss blood and die, but still.
If your character is in one of the few places in the world where they can get training in it, Systema’s also an option. In its modern form, it looks more like a subdual form, but it is quite lethal. Unfortunately, it also means your character needs to have come from someplace with a large Russian population. If the character is American, that means : Los Angeles, Chicago, New York, Seattle, or Miami.
Finally, if you’re willing to do some research on your own, there are a number of Ninjitsu schools in the US. Functionally, it’s not really that different from any other Japanese form, except that it hasn’t been defanged into a sport form yet. Just make sure, if you go this route, to make that completely clear to the reader.