Petty revenge stories are also welcome if you don't have an arch nemesis!
I used to know a guy who thought I was his arch nemesis.
I got too drunk and puked in his room one of the first days. My response from that point on throughout the rest of my years there was just to be ridiculously nice/normal to him and act like nothing happened. It made him so uncomfortable every single time.
There's a guy who tried to make out with my boyfriend literally the same night we had our first date. Apparently, he's really butthurt that boyfriend wouldn't go out with him and has tried talking him into going on a date with him even though we made our relationship official. I haven't met him yet, but when I do, I plan to be as civil and nice to him as I can be.
I'll kick this discussion off. My arch nemesis is my first ex. It's not the fact that she was a terrible partner, or that we broke up afterwards, because sometimes people aren't who you initially thought they were.
She is my arch nemesis because she did/is doing sociopathic things after the relationship ended. The biggest offense? Dating my roommate/best friend after saying she didn't want a relationship.