Most played cards in Underworld Breach:

2 Decks
Mox Amber
8 copies - 4 average
Grinding Station
8 copies - 4 average
Underworld Breach
8 copies - 4 average
Urza's Saga
8 copies - 4 average
Scalding Tarn
8 copies - 4 average
Unholy Heat
8 copies - 4 average
Mishra's Bauble
8 copies - 4 average
Emry, Lurker of the Loch
8 copies - 4 average
Dragon's Rage Channeler
8 copies - 4 average
Expressive Iteration
7 copies - 3.5 average
Engineered Explosives
6 copies - 3 average
Spell Pierce
6 copies - 3 average
Steam Vents
5 copies - 2.5 average
Wooded Foothills
4 copies - 2 average
Misty Rainforest
4 copies - 2 average
Lightning Bolt
4 copies - 2 average
Metallic Rebuke
4 copies - 2 average
Ragavan, Nimble Pilferer
4 copies - 2 average
Spreading Seas
3 copies - 1.5 average
Seasoned Pyromancer
3 copies - 1.5 average