I've done literally dozens of such experiments with single varietal pies since the early 2000's when heirlooms started to become more available and overall (of the apples included in this article) Northern Spy performs the best in both tasting panels and by my own accord. Not sure where the Granny Smith love is coming from; they generally turned to pie swamp when I used them, with little other than wine-y, acid to note for flavor.
That said, I almost never use Northern Spys because I much prefer Bramley's Seedling, which I can find from a great Orchard in Southern VT.

Other favorites:

-Baldwin (excellent all around pie apple that's making a big comeback with the advent of climate change; the tree is susceptible to frost damage)
-Rhode Island Greening (fabulous tart apple that blows Granny Smith away to the cider heap)
-Belle de Boskoop (hard to fine but worth it. The classic strudel apple, as well)
-Calville Blanc D'Hiver (a creamy, dreamy texture that isn't mushy)
-Gravenstein (summer apple that defies the stigma that the season doesn't produce good bakers)
-Spitzenberg (excellent flavor)
-Wolf River (you only need 3 or 4 of these monsters to make a pie and a very tasty one at that)

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