Within the U.S., where do you believe Standard English is spoken?

It depends what you mean by "Standard English". Americans in general use some words differently from how the same words are used in Britain, and Americans have words such as canyon; mesa; rodeo; lariat, which come from Spanish and are not used in Britain except in an American context.

Words used differently? "Biscuit" in Britain is what an American would call a "cookie". "Pavement"; USA: road surface for cars, Britain, walkway for pedestrians. "Creek"; USA: any small river, even hundreds of miles inland. Britain: last mile or so of a small river going into the sea.

I believe you mean General American (standard American English), and the answer is nowhere.

It is taught in schools, but nobody speaks it perfectly. Even when trying to be extremely formal, at least some grammar, vocabulary, and pronunciations deviate from General American depending on the speaker's native dialect (and often the speaker is not aware of it).

General American is based on dialects of several regions, not just one.

However, the mid-west states usually are closest to General American.

English does not have a single standard worldwide. The UK, Scotland within the UK, Australia, the USA, among others, all have their own formal standards, that vary in fairly minor ways.

Many people are under the false assumption that the dialects (or the dominating standard dialect) in England itself are the original English. The truth is that all living dialects everywhere are reasonably new. No dialects from 17th century England survives tot this day.

In fact, some dialects outside of the UK actually have preserved some features that are lost in modern UK dialects. One example: gotten, preserved in American English, has been replaced by got, in the UK. Of course, dialects outside of England also made many changes (but so have dialects in England).

The speech of Iowa is considered the standard for American English.

For British English, I assume the University towns Oxford and Cambridge.

Inside the British and Indian embassies in Washington, D.C., but not much of anywhere else.

Washington State

Nowhere outside a few university faculties - most Americans speak a bastardised version of Standard English.

In England.