The bottom right button should be the one affirming the default, taking you further:

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With the notable exception of toolboxes, all secondary windows should have at least one button. All button labels should use an imperative verb describing what it will do when clicked. The button with the suggested (or affirmative) action should be in the lower-right corner. This is probably the most discussed recommendation of the HIG.
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There really is no point in offering up Quit there anyway, it's not supposed to be something you do a lot. And there is already File>Quit and the window manager's close button.