The duel from "The Good, the Bad and the Ugly" certainly makes my top ten. But then, the whole movie start to finish is awesome, a masterpiece.
A scene I think absolutely jumps out from an otherwise very slow movie and should be in the running - the first Russian roulette scene from Deer Hunter, with Walkin and Dinero. The sheer intensity, the range of emotion - despair, rage, terror, hatred - the sheer will of Dinero in that scene, he wills them both through hell and their only path to survival. I wonder if either actor has ever been better.