Most played cards in Merfolks:

47 Decks
Force of Will
188 copies - 4 average
Master of the Pearl Trident
188 copies - 4 average
Aether Vial
188 copies - 4 average
Silvergill Adept
188 copies - 4 average
True-Name Nemesis
182 copies - 3.87 average
Chalice of the Void
170 copies - 3.62 average
Cavern of Souls
163 copies - 3.47 average
Lord of Atlantis
144 copies - 3.06 average
Thassa's Oracle
136 copies - 2.89 average
Paradigm Shift
136 copies - 2.89 average
Merfolk Trickster
132 copies - 2.81 average
Mutavault
103 copies - 2.19 average
Dismember
74 copies - 1.57 average
Grafdigger's Cage
57 copies - 1.21 average
Force of Negation
56 copies - 1.19 average
Brazen Borrower
54 copies - 1.15 average
Snow-Covered Island
50 copies - 1.06 average
Relic of Progenitus
41 copies - 0.87 average
Umezawa's Jitte
41 copies - 0.87 average
Phantasmal Image
41 copies - 0.87 average