Most played cards in Polymorph:

3 Decks
Polymorph
12 copies - 4 average
Bitterblossom
12 copies - 4 average
Thoughtseize
12 copies - 4 average
Preordain
12 copies - 4 average
Cryptic Command
12 copies - 4 average
Darkslick Shores
11 copies - 3.67 average
Mana Leak
11 copies - 3.67 average
Creeping Tar Pit
11 copies - 3.67 average
Mutavault
10 copies - 3.33 average
Drowned Catacomb
9 copies - 3 average
Wurmcoil Engine
7 copies - 2.33 average
Doom Blade
7 copies - 2.33 average
Ponder
7 copies - 2.33 average
Emrakul, the Aeons Torn
6 copies - 2 average
Jace, the Mind Sculptor
6 copies - 2 average
Sunken Ruins
5 copies - 1.67 average
Duress
5 copies - 1.67 average
Thopter Foundry
4 copies - 1.33 average
Smother
4 copies - 1.33 average
Scalding Tarn
4 copies - 1.33 average