I was watching a new short series that takes place in maine. Half the characters have a boston accent and it's honestly driving me mad. Maine is distinct from boston..

The show is wierd and Idk if I like it, and I def dont remember what it's called.

The 1920s to 1950s movie and radio voice wasn't just a random choice. It was done because the recording tech at the time was very limited and words had to be pronounced in a fairly specific and clear manner in order to be heard. Some accents had to be ommitted or suppressed, such as many Southern US accents, not out of disliking them, but simply because they didn't record well.

I truly find the mid Atlantic accent too be fascinating

Not to mention the myriad actors using a US accent(s). FWIW, I can't recall a movie/series that, when based in a region the actors know personally didn't do the local accent wonderfully. But, that's the rub: it's such an enormous, overpopulated populated landmass that it's bound to have innumerable dialects.

Although, perhaps what OP's asking is which, if any, is/are considered the common/baseline dialect of US English? 🤷🏼‍♂️