Any universal system is not going to do well at modeling reality well, since by definition it has to cover a lot of ground and therefore the details will get subsumed into general approaches. In particular, I’ve never found GURPS or HERO realistic; they do manage quite well at being believable, but it’s too easy to find specifics that just don’t match reality well.
I’d probably nominate ACES AND EIGHTS for it’s realistic portrayal of a very narrow niche: gun fighting in the ”Wild West”. Having played it twice, I am pretty convinced that the 3 hours it takes to resolve a 30 second conflict are very high in realism. As an example, whereas many games give a cover bonus, in this game you determine the physical location of each shot relative to one of several possible silhouettes you present to the attacker, and if that physical location is hidden behind a silhouette representing the cover, the bullet hits that cover. if not, it specifies precisely which part of you it hits. Thus, you can take cover behind a pillar and have a choice of what that pillar protects; but different shooters at you will see it protect a different part of your body.