Most of the noteworthy quirks in My Hero Academia are canon quirks designed by Kohei Horikoshi himself. But there are also some excellent non-canon quirks that appear in OVAs, movies, and even the few filler episodes in the anime itself. A few of them might even qualify as the best quirks in My Hero Academia!
Many of the best non-canon quirks come from Nine, a villain from the movie My Hero Academia: Heroes Rising, who obtained a copy of All For One that let him take multiple quirks from other people. But while Nine's quirks are powerful, they aren't the only ones worth mentioning. Flect Turn's Reflect, which appears in My Hero Academia: World Heroes' Mission, is nearly impossible to beat, while Selkie's Spotted Seal proves how powerful seals can be. Which of these is the strongest and best overall? That's up to you.