Most played cards in Grixis:

5 Decks
Augur of Bolas
20 copies - 4 average
Evolving Wilds
20 copies - 4 average
Pyroblast
20 copies - 4 average
Counterspell
20 copies - 4 average
Flame Slash
20 copies - 4 average
Izzet Boilerworks
18 copies - 3.6 average
Swiftwater Cliffs
16 copies - 3.2 average
Dismal Backwater
15 copies - 3 average
Terminate
13 copies - 2.6 average
Hydroblast
12 copies - 2.4 average
Dimir Aqueduct
12 copies - 2.4 average
Brainstorm
12 copies - 2.4 average
Electrickery
9 copies - 1.8 average
Duress
9 copies - 1.8 average
Gurmag Angler
9 copies - 1.8 average
Lightning Bolt
9 copies - 1.8 average
Ghastly Demise
9 copies - 1.8 average
Mystical Teachings
9 copies - 1.8 average
Curse of the Bloody Tome
9 copies - 1.8 average
Mulldrifter
8 copies - 1.6 average