Yzma has my favorite statline for a three-drop, as a 3/4 can often set up multiple favorable challenges in the early-to-midgame, allowing me to dominate the board
(You can of course skip the adventure, if you wish, and simply cast Kellan from your hand.) Decadent Dragon (and its adventure, Expensive Taste) is another card that is almost guaranteed to be a powerhouse in Constructed formats, is
Luck Bobblehead The Bobblehead collectibles from Fallout each get better the more you control
It gives lifegain on entry triggers and rewards you with card draw once you hit the life threshold, so routine board development keeps paying you back
Archfiend of Despair Atarka's Command Call In a Professional Erebos, God of the Dead Everlasting Torment Forsaken Wastes Giant Cindermaw Grievous Wound Grma Wormtongue Havoc Festival Knight of Dusk's Shadow Leyline of Punishment Quakebringer Rampaging Ferocidon Roiling Vortex Screaming Nemesis Skullcrack Stigma Lasher Sunspine Lynx The Lord of Pain Tibalt, Rakish Instigator Witch Hunt