I don't have an accent, which is unusual because common thinking is that everyone has an accent. Linguists differ on just who does and does not have an accent but their estimates are all very high (96%+) on just how many of us do have one. I do not. I learned this from someone who would know.
In 1968 I was a student in Germany. My professor there was a wonderful gentleman named Guenther Spaltmann. He was a native german, an accomplished artist and an truly gifted linguist. He had taught himself over a dozen languages; he was an interpreter at the peace negotiations after World War II, where each speech had to be translated live into English, German, French, Russian and Italian.
On our first day of german language class, Herr Doktor had us each introduce ourselves in german.
"Guten Morgen, Ich bin Tim Lavalli."
Dr. Spaltmann would then tell us where we were born or at least where we lived during our language acquisition years. When I spoke, he said: "You were probably raised west of Detroit but not in a close-in suburb or in eastern Iowa just over the Illinois state line." Apparently those are the two areas of the U.S. that have a complete absence of accent. What was called flat accent.
One might think this is a good quality for public speaking or commercial voice work, but in fact the absence of accent makes one's voice a bit monotonous (as in monotone). You need some inflection and rhythm. When I began teaching in L.A. in the 70s, I added a kind of a cadence to my speech pattern to shake off the monotone. I never really mastered an accent or a dialect but the irregular up and down changes in my voice kept the class awake, most of the time.
For an overwhelmingly cool map of dialects in the US
with clickable pronunciation guides - look here.