Saheeli, Sublime Artificer is one of the most exciting cards from War of the Spark simply in terms of the number of homes she has accessible to her. Both her abilities naturally lend themselves to a variety of different homes, and her mana cost being both cheap and readily castable helps support this too. More than anything, though, there's some real pedigree to her triggered ability:
One of these cards is restricted in Vintage, another spent a long time being a key threat in multiple Eternal formats, and the third was the card that arguably pushed Krark-Clan Ironworks over the top in Modern. Even Murmuring Mystic, the worst of this list, has been a powerful sideboard card in Standard ever since it was released. The ability to make 1/1s off various noncreature spells is just inherently powerful, letting you cascade value off whatever it is you are building your deck to want to do anyway.