Sorry but AoT is bad and all the other ones you mentioned are perfect examples for me to use. These series aren't necessarily a fad, they get recognition because of the fan base reading them. The majority of those readers are pre-teens to early teens,15-16, who think violence,vulgarity and nudity equate maturity.
So far Tokyo Ghoul and AgK have been better than AoT, I've went in blind though so I'm solely basing all 3 on the anime alone, TG has pretty nice art and animation as for plot idk what the plot is aside from there's flesh eating humans and a kid was made into one via organ transplant, they've still been able to pull some "didn't see that coming" moments like when the kids friend's friend ends up being a ghoul too. I'd say AgK and AoT were equal but AoT relied way more on the violence in the beginning to carry them, AgK had what 3 maybe up to 5 people die in the first episode? AoT had hundreds if not more.
Kids are always going to be attracted to stuff like that because it makes them feel a little bit older, it let's them feel like their doing something they shouldn't be by reading stuff with over the top antics whether it be violence,language or fan service. This new wave of manga is a result of newer generations constantly crying that previous works are too childish.