Flash's Most Underused Power Would Be Any Other Hero's Entire Identity
While the Flash is most known for running, his connection to the Speed Force grants several abilities that are enough for a whole other hero.
Despite being the Fastest Man Alive, the Flash has more than just the ability to run incredibly quickly. While his capacity to run at impossible speeds is his most famous power, writers have long worked hard at giving the character other applications to his super speed, from dimensional travel to superheated hands. In fact, a number of his ancillary powers could end up being a central power for any other hero.
Both of the most famous versions of the Flash - Barry Allen and Wally West - received their powers through accidents involving lightning hitting chemicals. However, in the 1990s at the height of Wally's popularity as the Flash, there was the introduction of a mysterious universal energy known as "the Speed Force" that opened up the cosmic implications of how the Flashes' powers work. This opened up the Scarlet Speedster to whole new avenues of powers tied to his speed to include the "infinite mass punch" and a rapid healing factor. Any single one of these applications could be sufficient on their own to make a superhero incredibly powerful.
However, some of these bizarre powers manifested long before the Speed Force was revealed in the comics. This includes the common Flash ability in the Silver Age to vibrate his molecules at high frequencies to gain intangibility and pass harmlessly through solid objects. Another such ability seen during the same time was Flash's ability to simulate flight by running on top of the water vapor of clouds. One ability that rarely gets mentioned is that Flash is able to vibrate his molecules to such a degree as to render himself functionally invisible, even while standing still - something he used to use casually, such as when he avoided a coworker in Flash #229 by Cary Bates, Irv Novick, and Frank McLaughlin. While it's seldom referenced today, just this ability alone would be enough to build a superhero career around (as DC did with the Invisible Kid from the Legion of Super-Heroes.)
Any one of these occasional feats of impossibility would be worthy of a superhero. Phasing through matter is the entire power set of characters like Kitty Pryde or the Fadeaway Man. The ability to heal quickly from injuries is the core superpower for characters like Deadpool and Wolverine. The capacity to punch at light speed would typically make a superhero a force to be reckoned with, and yet it's something Flash only draws on periodically.
While Flash often runs so fast that he's gone before anyone can see him, this vibrational invisibility was a whole other affair, allowing him to disappear even while standing "still." A variation of this was actually used by Flash's mentor, Jay Garrick, who would protect his secret identity by vibrating when photographed such that his features were always unclear. Flash has some of the most creative powers in comics, and while this has led to accusations that his abilities go too far, the fact that Flash possesses a power like invisibility - something which would be the foundation of another hero's identity - and hardly ever bothers to use it shows just how vast and varied his powered arsenal truly is.
This doesn't even take into account Speed Force abilities like when Impulse read the entire contents of the San Francisco Public Library in a single night with complete memory recall, or created speed constructs that could run through time, effectively making him a god. Those who tap into the Speed Force hold immense power at their fingertips, to the extent that superpowers others would kill for can end up falling by the wayside. All of this just goes to show how the Flash truly is one of the most powerful superheroes in the DC Universe.