Like most of us here, I have imagined this over and over in my mind a million times.
I have my revenge fantasy of wanting to hold him at gunpoint and tell him about every way the abuse has affected me... from age 4 on up. I dream of doing really sick stuff to him... putting the gun in his mouth to let him wonder when I'm going to shoot, beating the shit out of him and making him apologize for doing what he did over and over, make him plead with me for the pain to stop... etc. etc. etc. Basically, I dream of having control over him for once.
Then, there is a much quieter fantasy for me, but in some ways it is stronger than the revenge dream. If I knew it couldn't backfire, I would kill for him to come to me and tell me he knew what he did was wrong, and that he never should have done it, and that he did it because someone had done the same to him. This fantasy brings me to tears sometimes, it is so strong. I want to believe it wasn't just him being vicious, or getting pleasure out of hurting a little kid, or that it wasn't me that deserved it.
I know that's probably not exactly what you were looking for... for me, it's not about what I would say, but about wanting him to feel. Feel something... anything. Either feel the pain I endured until he understands what he truly did to me, or feel ashamed and humbled by his realizations and know that he worked to figure out why he did those things to me. Sometimes I think I would give my life just to have him tell me why he did it and say he was sorry for hurting me.
This a totally lame reference, but it stuck with me from the movie "Ever After." Drew Barrymore asks her stepmother if she ever loved her at all, even the littlest bit. And her stepmother says something like, "How can you love a pebble in your shoe?" And that is how I feel... despite the fact that my stepfather tortured me for four years of my life, I desperately wanted him to love me, to be the father I didn't have, I wanted to please him, and all I got was abuse and humiliation and pain.
Wow... didn't expect to bring all of that up tonight... hope this helps Irish.
-Sean