Mike D'antoni.
Some fans may boo but it's true.
He was not a big time coach when the Suns hired him. I am talking an experience NBA coach with a great reputation or a highly sought after coach everybody wants (assistant or college coach). Not that they come along that often.
The closest IMO is Cotton. The Suns have taken lots of chances with inexperienced guys or guys nobody really went after but a truly hot or big name coach?
It's the one thing I don't recall the Suns ever being very competitive in.