Most played cards in Mono-Black Devotion:

2 Decks
Desecration Demon
8 copies - 4 average
Mutavault
8 copies - 4 average
Thoughtseize
8 copies - 4 average
Pack Rat
8 copies - 4 average
Duress
8 copies - 4 average
Lifebane Zombie
7 copies - 3.5 average
Hero's Downfall
7 copies - 3.5 average
Ultimate Price
5 copies - 2.5 average
Dark Betrayal
4 copies - 2 average
Bile Blight
4 copies - 2 average
Doom Blade
4 copies - 2 average
Temple of Malady
4 copies - 2 average
Gray Merchant of Asphodel
4 copies - 2 average
Scavenging Ooze
4 copies - 2 average
Pharika's Cure
4 copies - 2 average
Abrupt Decay
4 copies - 2 average
Llanowar Wastes
4 copies - 2 average
Overgrown Tomb
4 copies - 2 average
Urborg, Tomb of Yawgmoth
3 copies - 1.5 average
Sign in Blood
3 copies - 1.5 average