I think they all should have been included, and in chronological order, which is why the cable TV "Saga" version is the best overall.
If you force me to name one, I'd choose the Genco deathbed visit because it provides perspective on Michael's coolness to his father, and on Tom's ascendency to consigliere. It also explains, in its own way, why Kay never appears with Vito ("Did your American girlfriend get home alright?"). Another of my faves is Gardiner Shaw and Francesca with Michael (the Don meets the WASP--an ingenious and compressed expression of Michael's yearning for "respectability"); Frankie feeding wine to Anthony on a spoon (you'd never, ever suspect him in the Tahoe shooting after that); and Neri forcing Klingman out of the hotel (Neri's largest chunk of dialog, plus that terrific Vegas show rehearsal--I bet FFC spent half a mil on that scene and then he cut it. ). All great stuff!
The young Roth scene isn't strong on dialog, but it's important in two ways: First, it establishes the basis for the relationship between Roth and the Corleones; second, it shows Vito as the undisputed boss--note how Clemenza, who bossed Vito around and subordinated him before the Fanucci murder, is now humble in his presence and even doffs his hat.