As a native speaker of Midwestern American English, I don't hear my accent as an "accent", naturally, but I know it's there. Any English speaker will recognize that I'm American as soon as I open my mouth and start talking English (I occasionally do better in other languages), and they'll probably recognize my accent as "Midwestern", if they've ever heard of it.
So it's not true that Midwesterners don't have accents; we do. As Schrödinger's Cat points out, everyone has an accent.
Possibly -- and here's the germ of truth in this myth -- it may be the fact that Midwestern English is the standard dialect for national broadcasting in the United States that people are referring to. Just as RP is standard on the BBC (with special exceptions for Northern dialects), Midwestern is standard in the US (with exceptions, mostly for Southern dialects).
That's all.
Don't believe everything you hear about English. In fact, generally it's a good idea not to believe anything you hear about English; there's an awful lot of nonsense around.
Best Answer
It's not that any particular accent is "more satisfactory" per se, but rather that a system will work better on accents broadly close to those that it has been trained on.
Speech recognition is essentially a statistical process: the system pulls in various acoustic cues and then tries to predict which word/sequence of words is the statistically most likely, given its "knowledge" of which acoustic cues in which combinations tend to indicate which speech sounds. It's difficult to say in advance for a particular utterance which feature(s) will be more prevalent in that prediction: it depends on the particular utterance and on the training data. So it's difficult to point to a specific feature of a particular accent in isolation that will be more or less satisfactory: the system is really much more complex than that. (I disagree from that point of view with Peter Shor's comment above: I don't think you can be so specific as to point to one particular feature and then say "that's the best accent" overall.)
If you're interested more in the subject, then I'd recommend introductions to speech recognition such as you'll find in: