Michael to Fredo:

"Fredo, you're my older brother, and I love you. But don't ever take sides with anyone against the family again. Ever."

I love that line. From the look on Michael's face, to his tone.. everything. He delivers it with a stone cold face.

And my pick for THE coldest line:

Michael to Fredo:

"I know it was you, Fredo. You broke my heart. You broke my heart."

I know not the most original choice. But that scene and that line, gave me chills.