As per correct grammar in English, the worst grammar I have heard in a people speaking English has been those of the UK, England specifically.
Although the accents are quite charming most English do not know a collective noun from a standard.. Oft time they will not conjugate verbs properly.
And much more.
It is true the past several decades the education system in the US has fallen greatly into an abyss of ignorance, however the language is basically more grammatically correct than that of the English system where the accent seems to outweigh the grammar.
When reading of the English actor, Michael Cane *Caine

, it was said when he attended college that he came out with an acceptable accent, no mention of acceptable grammar., and so it goes.
We do understand each other though, and this is 99 percent of speaking linguistically
Personally, I love to hear a good English accent, but I do recognize the errors in grammar..