The manga has taken a few liberties regarding what humans can achieve purely on physical and martial prowess but I still think the physical limitations of a human are heroes' biggest limitation when it comes to heroing. Sure, humans in this manga can achieve impossible things through sheer martial training and that allows them extraordinary feats when using quirks. But that doesn't mean heroes whose quirks enhance them don't develop this martial prowess. The physical capacity humans can get in the manga merely allows them to function as such. Imagine someone like eraserhead or stain but instead they were limited to what real life humans can accomplish physically.... The great eraserhead would be foiled by a 4 meter jump which he landed less than perfectly and caused permanent damage to his knee. And even then, people whose quirks do enhance them in some manner still have to develop their bodies in the same manner as people who are not enhanced by them. So the physical prowess normal humans can have don't really make up for this. There is also the issue that plenty of quirks aren't actually mor useful than a gun. With that in mind, I think the strongest quirks around are the following:
Frog form: Realistically speaking it's a perfect quirk for heroing. It gives the user enhanced mobility, increased reach with that tongue... I am not so sure of how having frog bones plays out into the strength (can she jump like that because her legs are unusually strong or does her bone density make up for that somehow) here but at least the speed the user can achieve should be significantly higher than a normal human could ever hope for. If her jumping capacity relates to strength then even now she basically has super strength. And by any logic she would be able to take significantly more damage than a normal human could. It's no OFA but enhanced speed, mobility, strength, range and endurance makes up an incredibly powerful quirk. The quirk won't really stop a bullet but at least it would be really damn hard to hit.
Dark shadow is quite spectacular as well given how it was recently used. Using it like an armor basically gives tokoyami all the benefits of having an enhancing quirk while also taking advantage of his shadow abilities. If he can ever use the power he used during the villain invasion of the camp them he will be a beast and easily a candidate for number 1 hero. Heck, it's plausible the shadow might even stop a bullet.
I would like to rate explosion or fire and ice higher but for the most part they provide little in terms of enhancing physical capacities. Which means that someone whose quirk enhances the user to beyond human levels would be capable of taking these guys with a single good attack. To put what I mean in context, imagine the fight between allmight and AFO. Allmight took an attack which sent him flying through the city like a champ. Would literally anyone else be able to do this? I sincerely doubt it. Endeavor would get have likely turned into a bloodstain far into the distance in the same scenario. Granted that either explosion or f&i can be used to compensate for a lack of enhancement and ice can be used as an armor but even that is far from perfect. Have frog form jumping at full speed at them to punch them and either would be out in that moment (assuming comparable base physical capacities). Also worth noting, either of this guys cannot deal with a gun. The most they could do is move around... Perhaps ice could put up a wall thick enough to stop a bullet but that only works if the user puts up the wall faster than what he is being shot at.
As for the phasing quirk, I am going to give it a low grade all around but there are a few things worth noting about it as well. It does not give the user any enhance strength or durability to speak of for one thing. Meaning anyone superhuman can potentially defeat him in one hit. For another thing, he does need to keep parts of his body solid for practical purposes like not being naked or perhaps even not phasing through the ground when he is trying to punch someone. But that only means he leaves bodyparts exposed to potentially superhuman damage when fighting. Perhaps something worth considering though is how fast he can phase through stuff. If he can swim through the ground for instance and do so at superhuman speed then his quirk would actually be incredibly useful. I still would not have him in the front lines but as a spying or sneak attack quirk it would be fantastic.
The electricity quirk has about the same potential as explosion or f&i but the same, if not more limitations. Given that it is electricity though perhaps it could work as a replacement for a gun. it's would hit much faster than bakugo's or todoroki's quirk.
The muscle enhancement quirk midoriya faced remains one of the stronger ones we have seen altogether. It is basically a lesser version of OFA.
Hardening is very good as well. It doesn't do much in the speed department as far as I can tell but the enhanced durability and strength it has makes it ideal for the front lines. Bakugo did win against this but not because his quirk was innately stronger but rather because he was the more skilled fighter. Or to put it another way, kirishima is a jackass. All kirishima has to do to beat bakugo at this point is improve his technique. I am not considering gadgets here though.
dental manipulation is IMO incredibly powerful. Clearly fast, grants a form of defense, grants a practical way to attack (impaling your enemy with teeth from a distance). Granted that it wouldn't be much use against a hardening quirk though. It's just teeth after all, if the quirk it is fighting has something that can resist teeth then this is over at least in that regard.
Now, this is not to say other quirks are useless. I am just considering this in terms of battle situations we could see in the manga. There are plenty of quirks which could be incredibly powerful in different scenarios while in the same scenarios the ones I mentioned would be useless.
