Most played cards in Grixis:

3 Decks
Watery Grave
12 copies - 4 average
Steam Vents
12 copies - 4 average
Blood Crypt
12 copies - 4 average
Temple of Epiphany
10 copies - 3.33 average
Ritual of Soot
9 copies - 3 average
Angrath's Rampage
8 copies - 2.67 average
Thought Erasure
8 copies - 2.67 average
Drawn from Dreams
8 copies - 2.67 average
Fires of Invention
8 copies - 2.67 average
Nicol Bolas, Dragon-God
7 copies - 2.33 average
Bedevil
6 copies - 2 average
Narset, Parter of Veils
6 copies - 2 average
Fae of Wishes
6 copies - 2 average
Fabled Passage
5 copies - 1.67 average
Ashiok, Dream Render
5 copies - 1.67 average
Tomebound Lich
4 copies - 1.33 average
Bond of Revival
4 copies - 1.33 average
Thrill of Possibility
4 copies - 1.33 average
Unmoored Ego
4 copies - 1.33 average
Drakuseth, Maw of Flames
4 copies - 1.33 average