All of them, at once. I test mainly in ChatGPT and Gemini because that's where most of your customers are, but good content earns citations across the board. You don't optimize for one engine and lose the rest.
AI keeps changing. Isn’t optimizing for it a waste?
The opposite. The engines change constantly, sure. What doesn’t change is that they all reward genuinely useful, well-structured content. Build that, and you stay visible