Most played cards in Grixis:

5 Decks
Wandering Fumarole
15 copies - 3 average
Spirebluff Canal
15 copies - 3 average
Negate
15 copies - 3 average
Sunken Hollow
14 copies - 2.8 average
Torrential Gearhulk
12 copies - 2.4 average
Unlicensed Disintegration
12 copies - 2.4 average
Fatal Push
11 copies - 2.2 average
Glimmer of Genius
11 copies - 2.2 average
Harnessed Lightning
10 copies - 2 average
Transgress the Mind
10 copies - 2 average
Incendiary Flow
9 copies - 1.8 average
Anticipate
9 copies - 1.8 average
Evolving Wilds
8 copies - 1.6 average
Aether Hub
8 copies - 1.6 average
Foreboding Ruins
7 copies - 1.4 average
Smoldering Marsh
7 copies - 1.4 average
Release the Gremlins
6 copies - 1.2 average
Ruinous Path
5 copies - 1 average
To the Slaughter
5 copies - 1 average
Fiery Temper
4 copies - 0.8 average