Most played cards in Grixis:

1 Decks
Ponder
4 copies - 4 average
Brainstorm
4 copies - 4 average
Force of Will
4 copies - 4 average
Scalding Tarn
4 copies - 4 average
Baleful Strix
4 copies - 4 average
Polluted Delta
4 copies - 4 average
Lightning Bolt
3 copies - 3 average
Kolaghan's Command
3 copies - 3 average
Snapcaster Mage
3 copies - 3 average
Underground Sea
3 copies - 3 average
Pyroblast
3 copies - 3 average
Thoughtseize
2 copies - 2 average
Plague Engineer
2 copies - 2 average
Surgical Extraction
2 copies - 2 average
Jace, the Mind Sculptor
2 copies - 2 average
Flusterstorm
2 copies - 2 average
Narset, Parter of Veils
2 copies - 2 average
Back to Basics
2 copies - 2 average
Hymn to Tourach
2 copies - 2 average
Volcanic Island
2 copies - 2 average