While not necessarily a deck-defining card, shes a crazy good 4-drop that can help you gain a lot of advantage in both mid and late game
Valakut decks Not only does it turn off the Molten Pinnacle You get a free shot at straight up killing Primeval Titan
This turns the deck more from a sacrificing and destroying-focused amass strategy into a spellslinging one, with Saruman amassing every time you cast a noncreature spell
For years, one of the running jokes in the Magic community was that R&D was secretly controlled by an alien brain in a jar named Gleemax
Kaldheim brought us Magda, Brazen OutlawMagda, Brazen Outlaw, which gave a power boost to other Dwarves you control, created Treasures, and let you sacrifice Treasures to search your library for an artifact or Dragon card-- instantly evocative of the Dwarven quest dramatized in The Hobbit