Nothing could beat Edwin van Cleef, a highly praised Rogue legendary, in terms of efficiency and power level. But The Boomsday Project has introduced a card that could easily compete with van Cleef. It's the Pogo-Hopper!
It starts as a 1/1 for two mana, but every time you play a copy of it, the card grows by +2/+2. You can keep it growing with the help of such cards as Shadowstep, Lab Recruiter, Mimic Pod, Elven Minstrel, and Vanish.
As you see, it basically works like Jade Golem mechanic, which produces minions that keep on growing to incredible sizes. Test this one out and make your opponents weep!