Most played cards in Grixis:

2 Decks
Lightning Bolt
8 copies - 4 average
Volcanic Island
6 copies - 3 average
Underground Sea
6 copies - 3 average
City of Brass
6 copies - 3 average
Red Elemental Blast
5 copies - 2.5 average
Power Artifact
4 copies - 2 average
Fireball
4 copies - 2 average
Serendib Efreet
4 copies - 2 average
Badlands
4 copies - 2 average
Dark Ritual
4 copies - 2 average
Blue Elemental Blast
4 copies - 2 average
Basalt Monolith
4 copies - 2 average
Hypnotic Specter
4 copies - 2 average
Counterspell
4 copies - 2 average
Power Sink
3 copies - 1.5 average
Blood Moon
3 copies - 1.5 average
Shatter
3 copies - 1.5 average
Underworld Dreams
3 copies - 1.5 average
Strip Mine
2 copies - 1 average
Juzam Djinn
2 copies - 1 average