Those of us from the Western United States would believe that we speak the "purest" form of American English (if such a thing exists)--I think this is what's being called "TV English" in the other post, although as a speaker of this dialect, my lip curls at such a designation.
However, even among us, we differ on pronunciations/word choice. A few examples: 1) the pop v. soda debate, which many argue has regional roots, although I've never been able to map it specifically; 2) bag being pronounced as either "baag" or "beg"; 3) tour being pronounced as either "too-er" or "tore".
As an aside, the irony I find is that I imagine the majority of Americans do NOT speak in our "generic" dialect, considering the density of population in other parts of the US (besides perhaps California).
However, I think this is one of the hallmarks of English. It does not have a regulating body like the French language. No one and everyone (thank you, J.M.) regulates English.