Well definitely want to use cards like Goldberry, Resourceful Defense, Nesting Grounds, and Aven Courier to shift our -1/-1 counters around so that we can abuse the untap effect of Xolatoyac, the Smiling Flood as a niche Seedborn Muse
In a strong second place is Sauron, the Lord of the Rings, which provides devastating value up front but costs a lot of mana and does surprisingly little after its ETB effect resolves
Flip-walkers, however, are a much more recent addition, first debuting in 2015s Magic Origins
Ketramose is plenty effective without the creature card type, but the ability gain life back makes this Orzhov card especially strong
Whenever your Dragons attack, Atarka gives them Double Strike
For example, Applejack lets a player summon their real-life My Little Pony toys as token creatures, while Fluttershy can lock down a creature from attacking or blocking as long as her controlling player is looking directly at the card