Most played cards in Grixis:

4 Decks
Crumbling Necropolis
16 copies - 4 average
Reflecting Pool
12 copies - 3 average
Cryptic Command
11 copies - 2.75 average
Cascade Bluffs
10 copies - 2.5 average
Infest
8 copies - 2 average
Sunken Ruins
8 copies - 2 average
Mulldrifter
8 copies - 2 average
Shriekmaw
8 copies - 2 average
Sedraxis Specter
8 copies - 2 average
Blightning
8 copies - 2 average
Demigod of Revenge
8 copies - 2 average
Makeshift Mannequin
7 copies - 1.75 average
Terminate
7 copies - 1.75 average
Cruel Ultimatum
7 copies - 1.75 average
Grixis Charm
6 copies - 1.5 average
Flashfreeze
5 copies - 1.25 average
Pyroclasm
5 copies - 1.25 average
Banefire
5 copies - 1.25 average
Fulminator Mage
4 copies - 1 average
Obelisk of Grixis
4 copies - 1 average