Most played cards in Grixis:

3 Decks
Force of Will
12 copies - 4 average
Ponder
12 copies - 4 average
Baleful Strix
12 copies - 4 average
Polluted Delta
12 copies - 4 average
Brainstorm
12 copies - 4 average
Scalding Tarn
9 copies - 3 average
Underground Sea
9 copies - 3 average
Snapcaster Mage
9 copies - 3 average
Surgical Extraction
8 copies - 2.67 average
Kolaghan's Command
8 copies - 2.67 average
Thoughtseize
8 copies - 2.67 average
Bloodstained Mire
7 copies - 2.33 average
Pyroblast
6 copies - 2 average
Plague Engineer
6 copies - 2 average
Volcanic Island
6 copies - 2 average
Badlands
6 copies - 2 average
Fatal Push
6 copies - 2 average
Hymn to Tourach
6 copies - 2 average
Jace, the Mind Sculptor
6 copies - 2 average
Lightning Bolt
5 copies - 1.67 average