The most utilized card during Arena’s recent Mythic Championship, was Once Upon a Time from Throne of Eldraine! The utility of this card is off the charts and I think it fits into almost any green deck. First off, this is a super effective mana fixing card. In an opening hand, it turns into a land without any cost, if it’s the first card you cast.This is really good when you draw first. If you’re going second, I usually wait until after the draw to see if I draw into a land on the first draw. But then, if you have already have enough land to start, it immediately turns into a creature card to try and fit into your ramp up…and you can cast it on your opponents turn before you draw…which is the second real benefit…the deck thinning. Once you have your land, you usually don’t want to flood, so it helps reduce the actual mana ratio in the deck itself. After you’ve played though, just a green and colorless is a minimal cost to search for another land or another creature. Once Upon a Time is already a 4-pack in most of my Standard green decks, and I suspect it’s going to become a staple card in Modern and Pioneer.
Get Once Upon a Time for your deck here!
Update Nov. 18, 2019: Once Upon a Time is banned in Standard.