In My Hero Academia, heroes are defined by their actions, not the brute strength associated with their powers (or quirks, as the show calls them). While the main characters typically exhibit the more incredible abilities, underrated skills tend to get lost in the shuffle. Many of these quirks are more useful than visually impressive.
Edgeshot's Foldabody allows him to make his body so sharp that it can cut like a sword, or so thin that he can slip through cracks, all at the speed of sound. While it's easy to see why this quirk is incredible, Edgeshot himself is pretty forgettable. Other underrated quirks have uses that may not seem immediately obvious – Shoji Mezo's Dupli-Arms can do a whole lot more than hold four coffee mugs at once.
Because many of these underrated quirks appear in the manga, this list may contain spoilers for fans who aren't caught up.
Chiyo, the school nurse at UA High School, has a quirk that's easy to overlook, but is actually essential if the whole school isn't going to end up dead. Her quirk, Heal, engages a target's immune system to help them heal rapidly from injuries that might otherwise take months to recover from, or might never heal at all.
The only drawback is how it works. She essentially uses the body's own resources to heal the injury. If the target person isn't strong enough to withstand the healing process, it might need to be spread out over multiple sessions. Still, Chiyo's abilities are the reason Izuku makes it past breaking his arms the first time he uses his quirk.
Without her, he would miss months of hero training while recovering.
Foldabody is a quirk that allows its user to manipulate their own flesh with precision. Edgeshot is able to make his body extremely thin or unbelievably sharp. This allows him to slip through small cracks, granting him access to any location he wants. It also allows him to use his own body as a knife or a sword.
What's more, he can make these changes faster than the speed of sound. No wonder he's the fourth most powerful hero.
Using Confession, Shin Nemoto can force anyone he touches to answer a question truthfully. He uses this to learn about the motives and secrets of both his enemies and his allies. This helps him determine how to defeat his foes, psychologically torment them as he deems necessary, and gather information from them, while ascertaining who he can trust.
It's an impressive ability to definitively prove key information without the need for deduction or guesswork.
Atsuhiro Sako is only seen using his quirk, Compress, for a singular purpose: compressing Bakugo into a tiny marble shape so he can easily kidnap him, while ensuring he remains unharmed. While this is undoubtedly useful for his villainous purposes, it's also great for things like carrying heavy weaponry, or unconscious or injured comrades.
Outside of fighting, it's great for carrying heavy objects. With Compress, one could technically carry the entire contents of their house in a single bag, making moving a breeze.