Hey, I'm new to the board also...
I can't believe no one mentioned Senator Geary stumbling through Anthony's name. I thought that was pretty funny. Also Luca Brasi practicing what he would say to Vito, then messing up in the end: "Don Corleone. I am honored... and grateful... that you have invited me to your daughter's wedding......... on the day of your daughter's wedding..." That always cracks me up.
I also think it's amusing during the christening scene, the way organ music and Michael's professing to renounce the devil provide the background for a montage of bloody murders.