Best Revenge?

I agree with ShotaSidePart about Kenja no Ai.  Great, twisted revenge movie.  I loved it as well, it's pretty wild.

I just finished watching another pretty great revenge movie.  Not sure if it'd be considered one of the "best revenge" ones, but I was so impressed by it, that while it's fresh in my mind, I'll list it here.  
Liverleaf   Definitely do not take the "Gore" and "Extreme Violence" tags lightly.  It was well done.