Here is another card that has been available in Legacy but not Modern. Now this reprint from the Urza's Legacy set finally finds its way into the Modern format.
Unearth can bring back creatures like Death's Shadow, Tarmogoyf, Scavenging Ooze, Snapcaster Mage, Dark Confidant, Fulminator Mage, and many others. All these cards are extremely valuable and replaying them for just one mana from your graveyard would be quite incredible.
On top of that, it has cycling, which makes it a valuable addition to any maindeck that requires an early draw.
All this makes Unearth the most valuable common card in the Modern Horizons set.