Most played cards in Grixis:

7 Decks
Force of Will
20 copies - 2.86 average
Pyroblast
17 copies - 2.43 average
Surgical Extraction
16 copies - 2.29 average
Volcanic Island
15 copies - 2.14 average
Scalding Tarn
14 copies - 2 average
Flusterstorm
14 copies - 2 average
Lightning Bolt
13 copies - 1.86 average
By Force
12 copies - 1.71 average
Snapcaster Mage
10 copies - 1.43 average
Dack Fayden
10 copies - 1.43 average
Underground Sea
10 copies - 1.43 average
Polluted Delta
9 copies - 1.29 average
Jace, the Mind Sculptor
8 copies - 1.14 average
Preordain
8 copies - 1.14 average
The Tabernacle at Pendrell Vale
6 copies - 0.86 average
Force of Negation
6 copies - 0.86 average
Hurkyl's Recall
6 copies - 0.86 average
Flooded Strand
6 copies - 0.86 average
Mox Ruby
5 copies - 0.71 average
Mox Pearl
5 copies - 0.71 average