Comparison
When I look at the design methods from Figure 1 next to each other, I notice that they overlap a lot, but that they still each put the focus somewhere else. Brainstorming is mostly about opening up the design space and coming up with many possible ideas before choosing a direction. Rapid prototyping comes after that more naturally, because it is about turning ideas into something tangible as quickly as possible, so you can see what works and what does not. Iterative design feels broader to me, because it is less one specific action and more a repeating cycle of making, testing, reflecting, and improving.
Co-design stands out because it changes who is part of the design process. Instead of designing something only by yourself, you involve users or other stakeholders more actively. Research through design is different again, because the goal is not only to make a final design, but also to learn something through the process of designing and making. Critical design also has a different role, because it is often less focused on solving a direct problem and more on questioning assumptions, provoking discussion, or making people think differently about a topic.
So even though these methods are clearly related, I see them as doing different kinds of work in a design process. Some methods help generate ideas, some help test and improve them, and others help involve people, build knowledge, or challenge existing views. That is also why they can work well together instead of replacing each other.
Figure 1. Mind map showing the relations and differences between six design methods.