Most played cards in Merfolks:

5 Decks
Merfolk of the Pearl Trident
20 copies - 4 average
Lord of Atlantis
20 copies - 4 average
Counterspell
20 copies - 4 average
Phantasmal Terrain
17 copies - 3.4 average
Blue Elemental Blast
17 copies - 3.4 average
Mishra's Factory
17 copies - 3.4 average
Serendib Efreet
16 copies - 3.2 average
Unsummon
14 copies - 2.8 average
River Merfolk
14 copies - 2.8 average
Psionic Blast
14 copies - 2.8 average
Energy Flux
13 copies - 2.6 average
Psychic Purge
11 copies - 2.2 average
Boomerang
8 copies - 1.6 average
Flying Men
8 copies - 1.6 average
Strip Mine
8 copies - 1.6 average
Control Magic
7 copies - 1.4 average
Time Walk
5 copies - 1 average
Black Lotus
4 copies - 0.8 average
Ghost Ship
4 copies - 0.8 average
Black Vise
4 copies - 0.8 average