A "creature source" is a permanent, spell, or card in any zone with the card type "creature." For example, Echo's ability can target a cycling ability you've activated if the discarded card is a creature card
Honorable mentions There are many, many cards that have seen word reductions in the dozens
Of all the praetors, Jin-Gitaxias is the most concerned with converting the face of the world formerly known as Mirrodin to its Phyrexian ideal, from the smallest organism to the greatest sea
Well, thats what Brokers Ascendancy does in every one of your end steps
But in a meta game with good enchantments to destroy like Wedding Announcement, enchantment creatures, and sagas, it suddenly becomes a worthy consideration
Giving you the chance for one free recursion every one of your upkeeps from any graveyard is pretty rock-tastic, and thus, the high ranking on today's list