I think Rith, the Awakener is the best of the cycle
While a little confusing on paper, this boils down to allowing the repeated use of exhaust abilities, turning them from a once-per-game affair into a once-per-turn situation, allowing for many of the more powerful abilities using this keyword to be activated again and again
Creatures like Piper of the Swarm, Totentanz, Swarm Piper, and Ogre Slumlord can ensure your field is consistently full of rats, and the nonlegendary Piper even lets you use them to steal your opponents creatures
Heck, even some of the planes that don't give this vibe still lent well to the concepts at play, like how Dominaria has since become a plane all about history
Transform cards become even more cumbersome when their flip condition is like threading the needle