Most played cards in Grixis:

3 Decks
Polluted Delta
12 copies - 4 average
Bloodstained Mire
11 copies - 3.67 average
Sunken Hollow
8 copies - 2.67 average
Spirebluff Canal
7 copies - 2.33 average
Dig Through Time
7 copies - 2.33 average
Collective Brutality
7 copies - 2.33 average
Jace, Vryn's Prodigy
6 copies - 2 average
Kolaghan's Command
6 copies - 2 average
Smoldering Marsh
6 copies - 2 average
Flooded Strand
5 copies - 1.67 average
Torrential Gearhulk
5 copies - 1.67 average
Void Shatter
4 copies - 1.33 average
Unlicensed Disintegration
4 copies - 1.33 average
Duress
4 copies - 1.33 average
Fevered Visions
4 copies - 1.33 average
Stormchaser Mage
4 copies - 1.33 average
Grasp of Darkness
4 copies - 1.33 average
Slip Through Space
4 copies - 1.33 average
Dispel
4 copies - 1.33 average
Lightning Strike
4 copies - 1.33 average