Most played cards in Machinehead:

2 Decks
Duress
8 copies - 4 average
Bloodstained Mire
8 copies - 4 average
Sulfurous Springs
8 copies - 4 average
Cabal Therapy
8 copies - 4 average
Mishra's Factory
8 copies - 4 average
Lightning Bolt
7 copies - 3.5 average
Lavaborn Muse
7 copies - 3.5 average
Ravenous Rats
7 copies - 3.5 average
Terminate
7 copies - 3.5 average
Burning Wish
6 copies - 3 average
Withered Wretch
6 copies - 3 average
Grim Lavamancer
6 copies - 3 average
Engineered Plague
6 copies - 3 average
Red Elemental Blast
5 copies - 2.5 average
Hypnotic Specter
4 copies - 2 average
The Rack
3 copies - 1.5 average
Dark Ritual
3 copies - 1.5 average
Urza's Bauble
3 copies - 1.5 average
Tainted Peak
3 copies - 1.5 average
Meltdown
2 copies - 1 average