Most played cards in Grixis:

5 Decks
Lightning Bolt
20 copies - 4 average
Volcanic Island
14 copies - 2.8 average
Underground Sea
14 copies - 2.8 average
Red Elemental Blast
14 copies - 2.8 average
Badlands
14 copies - 2.8 average
City of Brass
12 copies - 2.4 average
Mishra's Factory
12 copies - 2.4 average
Counterspell
12 copies - 2.4 average
Blue Elemental Blast
12 copies - 2.4 average
Serendib Efreet
11 copies - 2.2 average
Shatter
10 copies - 2 average
Fireball
8 copies - 1.6 average
Psionic Blast
8 copies - 1.6 average
Chain Lightning
8 copies - 1.6 average
Hypnotic Specter
8 copies - 1.6 average
Dark Ritual
8 copies - 1.6 average
Control Magic
6 copies - 1.2 average
Sinkhole
6 copies - 1.2 average
Blood Moon
6 copies - 1.2 average
Underworld Dreams
6 copies - 1.2 average