Most played cards in Grixis:

1 Decks
Faithless Looting
4 copies - 4 average
Bedlam Reveler
4 copies - 4 average
Inquisition of Kozilek
4 copies - 4 average
Lingering Souls
4 copies - 4 average
Leyline of the Void
4 copies - 4 average
Bloodstained Mire
4 copies - 4 average
Lightning Bolt
4 copies - 4 average
Blackcleave Cliffs
4 copies - 4 average
Young Pyromancer
4 copies - 4 average
Kolaghan's Command
3 copies - 3 average
Blood Moon
3 copies - 3 average
Wooded Foothills
3 copies - 3 average
Thoughtseize
3 copies - 3 average
Forked Bolt
2 copies - 2 average
Dragon's Claw
2 copies - 2 average
Monastery Swiftspear
2 copies - 2 average
Lightning Helix
2 copies - 2 average
Sacred Foundry
2 copies - 2 average
Wear // Tear
2 copies - 2 average
Pithing Needle
2 copies - 2 average