The most obvious would be small universal systems, although (a) the importance of that is subjective and (b) much of the credit goes to others (e.g. Matthew Cook for proving Rule 110, Alex Smith for the 2,3 Turing Machine)
But the critique I've heard is that, in fact, folks like Cook and Smith deserve the credit and Wolfram played more like a coordinating role.
The attention ecosystems of entertainment, science, computing seem to favor a model people who are good at self-promoting a particular "brand" of work that really represents the labor of a much larger group of people. This simplification is useful for a couple of reasons: it nurtures the ego of the brand-manager, which almost has to be outsize to play this role. It is a simple enough story for the public to understand, and efficiently creates a large "catchment area" for people to become interested in the brand. And the brand managers can sometimes be good coordinators.
Wolfram did speculate that 110 would be Turing complete, although others proved it. Particularly in the Maths, an insightful but as-yet unproved assertion can be at least as valuable as its proof (hence all the Mathematicians who are justifiably famous for a conjecture).
I'd also suggest that cellular automata are now an important area of theoretical Comp Sci (and a number of related fields). Wolfram's contribution in that area compares quite favorably with the pioneering work by Von Neumann and Conway, providing a lot of the foundation for systematic study of those types of system.