Rise of the Eldrazi (2010) The Eldrazis were ancient creatures that predated the colors of magic, so they were represented by colorless cards, with transparent frames
You can leverage this as a two-for-one quite easily, either as a surprise combat trick that eats a creature or a counter-bounce spell that mimics Cryptic Command
Even killing it wont do your opponents much good since it can easily flip back over into a creature when you attack with at least three creatures
They also had a 3-color cycling cost, associated with the basic land types they could fetch
One of the biggest challenges of building around Naban, Dean of Iteration is that if you are building a Naban deck, you'll pretty much have to play four copies, but since Naban is legendary, this will lead to some clunky dead draws
Tekuthal, Inquiry Dominus (EOC) $10.84 Everybody loves to double stuff, and proliferate is also a fan favorite mechanic