I also agree that Marx is a thinker who altered the course of the world and I see where you are coming from regarding the moral judgement on his absence.

As a counterpoint, what would Marx’s great intellectual achievement be, and could it stand up to the early capitalists like Smith?

What comes to mind is the Labour Theory of Value, and I would say it is a strong candidate for sure. Whether it figures as a key human intellectual achievement is definitely at best borderline compared to the other exemplars on this list.