Most played cards in Grixis:

5 Decks
Lightning Bolt
20 copies - 4 average
Scalding Tarn
20 copies - 4 average
Crumbling Necropolis
20 copies - 4 average
Spreading Seas
20 copies - 4 average
Creeping Tar Pit
19 copies - 3.8 average
Terminate
17 copies - 3.4 average
Jace, the Mind Sculptor
16 copies - 3.2 average
Countersquall
13 copies - 2.6 average
Siege-Gang Commander
12 copies - 2.4 average
Drowned Catacomb
12 copies - 2.4 average
Sedraxis Specter
12 copies - 2.4 average
Dragonskull Summit
12 copies - 2.4 average
Blightning
12 copies - 2.4 average
Consuming Vapors
10 copies - 2 average
Cruel Ultimatum
10 copies - 2 average
Flashfreeze
9 copies - 1.8 average
8 copies - 1.6 average
Into the Roil
7 copies - 1.4 average
Jace Beleren
7 copies - 1.4 average
Earthquake
7 copies - 1.4 average