I am the most powerful, a group of people.
My opposite was once my rival, able to change between others.
We can both be found in the same surroundings, often doing very different things.
For example, one of us could be relaxed while the other is making things happen.
One could be underneath while the other is making connections.
One could be in court while the other is getting help from equipment.
One can bring power while the other shows disease.
On the radio, one of us could be reading while the other gives chase.
Normally, though, our interests overlap.
What am I and what is my opposite?
Bonus: where else can we both be found?
As suggested by rand al'thor, the answer is a letter or small group of letters rather than a word.
Each 1/2 line gives a single word that is a clue towards the final answer.