I think my best friend cheated on her boyfriends... with me.
What makes you think that that is what happened?
She makes me feel so much better than them all, like I'm a king or something.
You must be flattered that she treated you that way, but it is still not acceptable for her to cheat in any way. She did this behind her boyfriends backs without telling them she wants to break up. My advice would be to let her go, she  will end up cheating on you as well.
Let's say I want to seek revenge, you know, as a payback for dumping me back in high school. What should I do to get her back for cheating on her boyfriend?
You should do nothing. I think that you should just tell her that it is not okay that she cheated on her partner.