Most played cards in Mystic Forge Combo:

2 Decks
Karn, the Great Creator
8 copies - 4 average
The One Ring
8 copies - 4 average
Ancient Tomb
8 copies - 4 average
Lotus Petal
8 copies - 4 average
Grim Monolith
8 copies - 4 average
Defense Grid
8 copies - 4 average
Manifold Key
8 copies - 4 average
Mystic Forge
7 copies - 3.5 average
Urza's Saga
7 copies - 3.5 average
Voltaic Key
7 copies - 3.5 average
Basalt Monolith
5 copies - 2.5 average
Paradox Engine
5 copies - 2.5 average
Mox Opal
5 copies - 2.5 average
Faerie Macabre
4 copies - 2 average
Dismember
4 copies - 2 average
Glaring Fleshraker
4 copies - 2 average
Leyline of the Void
4 copies - 2 average
Urza's Workshop
4 copies - 2 average
Planar Nexus
4 copies - 2 average
City of Traitors
4 copies - 2 average