To name just one would be a crime!
In GFI it has to be the Sollozzo killing. Not just the actual scene at Louis', but the whole buildup starting when they are sitting around eating the Chinese food at the Corleone compound. It just has great tension and the music is wonderful. The sounds of the subway overhead are brilliant!
GFII- The "stepped over" scene with Michael and Fredo. I love the way it's shot starting with how everything in the foreground(including Michael and Fredo) are blacked out. Outside the beautiful lake with the snow hitting the windows. Fredo letting out all his emotions on something that has obviously been building for a while.
GFIII- Agreement with a lot of others around here, that the ending on the steps of the opera house, to the flashbacks of Michael dancing with Mary, Kay, and Apollonia.