I have to admit to not having read Leonard, so you're in good company. And McBain was one of the greats. Funny, how it can't really be narrowed to one, isn't it?!
Off hand, I have to admit that I can't think of any mystery writer as a stylist. The ones that sell well are often former journalists. Their style much resembles the stripped-down, simple newspaper style. It tends to be effective for crime "reporting" and makes it easy for readers to follow the story.
Occasionally, you see a little experimentation, as in Ken Bruen's Jack Taylor series. His poem-like lists of nouns, for example, are usually effective and appropriate to the Irish mood and character. Experimentation is good.
For myself, I have discovered that readers pause with pleasure over the pretty stuff, so perhaps there is room in mysteries for some fancy language.
Thank you. that's really making me think. I do pause over some things. But then I do it over stuff that's just so pared down and minimalist but shocking. Like some of James ellroy. thanks so much.