Crimson Kobolds

Pay nothing, get a permanent: that is the entire pitch, and it is also the most structurally loaded square on the design grid. A 0/1 body at mana value zero is the substrate every storm engine, every cost-reducer, and every "creatures you control" payoff has to be priced against. The body blocks once, dies, and is otherwise beside the point; the slot is everything. The cards that grew up around the Kobolds tell the story: Kher Keep makes more of them, Prossh, Skyraider of Kher counts them as sacrifice fodder, and a generation of "creatures you control" payoffs gets a free unit to multiply. They were built around Kobolds because Kobolds are the cheapest possible piece of creature, and the design tax for that has been paid downstream for decades. Every printing that touches "whenever a creature enters" or "creatures you control" has to look back at this card and confirm the math still holds. Free creatures are a design line Wizards has spent a long time quietly walling off, and this card is part of why.