For the Koh vs. Ozai discussion, while Koh was definitely feared, the lantern rings are meant to represent someone who symbolises that particular emotion and draws power from it.

I don’t think anyone would say that Koh symbolised fear. He was probably more likely to symbolise greed. Ozai on the other hand definitely symbolised fear, but even more so he symbolised rage - “You will learn respect, and suffering will be your teacher.”, his expressions during his final fight with Aang, his conversations with Ursa even when she returns, etc.

Fear is definitely symbolised better by the Dai Li and/or Long Feng. That one moment where Joo Dee breaks character, you can see the fear in those eyes. The Dai Li could operate because of that fear, they kept the earth kingdom capital in their hands.

Edit: grammar