Most played cards in Grixis:

1 Decks
Scalding Tarn
4 copies - 4 average
Remand
4 copies - 4 average
Opt
4 copies - 4 average
Thing in the Ice
4 copies - 4 average
Snapcaster Mage
4 copies - 4 average
Polluted Delta
3 copies - 3 average
Surgical Extraction
3 copies - 3 average
Cryptic Command
3 copies - 3 average
Lightning Bolt
3 copies - 3 average
Blood Moon
2 copies - 2 average
Sulfur Falls
2 copies - 2 average
Young Pyromancer
2 copies - 2 average
Damping Sphere
2 copies - 2 average
Anger of the Gods
2 copies - 2 average
Steam Vents
2 copies - 2 average
Field of Ruin
2 copies - 2 average
Sleight of Hand
2 copies - 2 average
Kolaghan's Command
2 copies - 2 average
Fatal Push
2 copies - 2 average
Jace, the Mind Sculptor
2 copies - 2 average