Most Helpful Guy
The best revenge is living well. Going out of your way to make him sad is just gonna drag you down too.
But if you happened to find a much more handsome, less-cheaty boyfriend, I'm sure that it wouldn't take too much effort to make sure he sees photos of you two, let him know how great things are without him.2
Most Helpful Girl
Move on, act like you didn't even care and then try to improve your life (try harder at work, find new skills or talents, get healthier, start working out, )
Showing him that you never cared and that your life is better without him is the best revenge