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
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
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
Third-party research gives us a pretty good idea of the days and times of the week where there is the highest activity (and highest sale value) for items sold on major platforms like eBay