The Borg are the best from a narrative point of view.
They are a harsh mirror held up to the idealism of the Federation. They preach unity, peace and the importance of a common purpose. All it costs is your individuality, your freedom and your consent.
The Dominion tread this same path, but they weren't handled as effectively. If they had played up the nature of the Dominion as an alliance of many species, each filling their purpose for the greater whole, and how that can be twisted, it would have been more effective.
Q takes another tack and is more successful, simply because the question is more explicit: Is the Federation, and specifically Humanity, worthy of existence? In the presence of a god, should you have to justify your right to be, and could you even do so if you wanted to?
Star Trek is at it's best when it is engaged in a battle of ideals, when the principles of the societies it presents are attacked.