I think it is his best, and one of the few worth reading. If you've read one, you've read most of them. So you should read the best ones, and this is probably the best of the best.
It is really a sequel to The Weapon Shops of Isher. But I read this one first, and it stands alone quite well. The protagonist, the Van Vogtian superman, is Robert Hedrock. He was made immortal through an accident. But being extremely altruistic, he has spent over five thousand years trying to duplicate the accident, and thus make immortality available to all mankind.
Being of a libertarian nature, Hedrock founded the Weapon Shops. In these shops, common citizens can acquire powerful weapons that can only be used in self-defense, to prevent them from being oppressed. Furthermore, we find out in this novel, that he also is responsible for the ruling Imperial dynasty. Thus, he is involved in both ends of the government.
The crux of the story is Hedrock's interaction with Innelda, the young, beautiful, intelligent, strong-willed empress. A strong love-hate relationship here. He finds out that the Empire is close to discovering an interstellar space drive. Innelda sees this as a threat to Imperial power. Hedrock sees it as a way for mankind to progress and grow. Conflict is inevitable. Almost as an aside, aliens appear on the scene to evaluate humankind.
The challenge here is to make Hedrock seem as intelligent as he is supposed to be. An IQ of around 278 is mentioned, so high as to almost be meaningless (in another story, I believe van Vogt mentions an IQ of around 800. Completely meaningless.). But van Vogt brings it off very well. From his actions and wit, you can almost believe that the guy is super-intelligent. He keeps one step ahead of both the Weapon Shops and the Empire in terms of technology, because he is so smart, and to protect his identity. Along the way, we are treated to great ideas in terms of dimensional travel, limited time travel, and "magnifiers" to make things giant-sized, although at a huge cost in aging (which is less of a problem if you are an immortal).
Sure parts of it are stilted and melodramatic. But gosh, the sweep of it. The intellect of the main character. The gadgetry. For a story from the forties, it has really held up. Highly recommended.