Most played cards in Grixis:

3 Decks
Lightning Bolt
12 copies - 4 average
Volcanic Island
10 copies - 3.33 average
Red Elemental Blast
8 copies - 2.67 average
Serendib Efreet
8 copies - 2.67 average
Counterspell
8 copies - 2.67 average
City of Brass
8 copies - 2.67 average
Badlands
7 copies - 2.33 average
Blood Moon
6 copies - 2 average
Chain Lightning
6 copies - 2 average
Blue Elemental Blast
6 copies - 2 average
Underground Sea
6 copies - 2 average
Fireball
5 copies - 1.67 average
Mishra's Factory
5 copies - 1.67 average
Shatter
5 copies - 1.67 average
Power Artifact
4 copies - 1.33 average
Psionic Blast
4 copies - 1.33 average
Basalt Monolith
4 copies - 1.33 average
Dark Ritual
4 copies - 1.33 average
Hypnotic Specter
4 copies - 1.33 average
Power Sink
3 copies - 1 average