I really liked the whole Vito whacking/Luca garroted/Tom picked up by Sollozzo sequence, but my fav individual scene in Part 1, is the Restaurant Shooting.

In GF2, when it pans to Michael standing in the window, looking out on the lake, and you hear Al Neri shoot Fredo, then a few seconds later, you hear a bird screetch, as you see Neri stand up, and then it pans back to a drained Michael, as he hangs his head. Probably the most powerful scene in the whole Trilogy.