In what capacity are you asking “fastest” in its literal interpretation in pure speed: All Might, Endeavor, Hawks, Izuku Midoriya, and Tenya Iida are the fastest combatants.
However, reaction time/reaction speed is another story. Of course Midoriya & All Might are still on top due to the sheer multiplier that is One For All. But, Eraser, Shoto Todoroki, Katsuki Bakugo, Hawks, and Stain are all upper-tier characters with extreme reactions times as well.
The fastest hero in outright speed is the Winged Hero: Hawks
With Midoriya (100%) and All Might the #1 Hero coming in a close second place:
Finally, Endeavor using Flashfire is going to be just below All Might in third place behind One For All:
Now, Reaction time is another gamble but based on the scenarios we’ve seen: All Might is going to be on top due to him reacting in an instant when Nomu speed-blitzed Bakugo during the USJ attack.
Second place would likely go to Hawks due to his quirk amplifying his sense to an extreme level especially with his hypersensitive wings.
Next, Eraser, him dodging Dabi’s attack in the time it took for Dabi to ignite his flames Eraser had activated his quirk and jumped out of the blast radius.
After Aizawa, we’re looking at mainly students on the top of this list with either Mirio Togata or Stain; Stain due to his capabilities in predicting his opponents moves and being able to deduce Midoriya’s plan when they fought in the alley.
Some comments seem to think All Might is faster, he is capable of moving 5 kilometers in 30 seconds during the Hideout Raid Arc, which translates to 600 km/hr where as Hawks moved in the blink of an eye from the top of a skyscraper to Endeavor’s battle in 0.1–0.3 seconds. Assuming it’s a decent sized skyscraper in say the Tokyo prefecture: I’ll assume it’s only 20 stories (quite low all things considered) assuming 12 feet or so/floor we have 240 feet or 80 yards.
80 yards in 0.3 seconds translates to 878 Kilometers/Hour. That is over 200 km/hour faster than All Might all the while Hawks believes he could’ve easily gone faster to Endeavor’s aid.
That being said All For One also said that All Might has slowed since their first confrontation. Giving All Might SOME leeway they would be close around 850 km/hour regularly.
In conclusion Hawks is the fastest hero in My Hero Academia by a landslide. All Might was praised for his absurd strength, whereas Hawks is known for his S-class speed in the data books. Don’t doubt my boy Hawks.