Selvala, Heart of the Wilds

The mana ability is the part that broke containment: tap her with a large enough creature in play and she produces mana equal to the greatest power among your board, in any combination of colors, untethered from her own modest body. That scaling is the whole engine. A single hexproof fatty or an Eldrazi turns her output from a trickle into a flood, and because the mana comes in any combination of colors, she has anchored some of the most notorious infinite-mana loops in singleton formats, the kind that pair a haste enabler with a creature whose tap or enters-the-battlefield trigger feeds back into her. On her own she is finite: she taps for two mana (her own power), useful fixing and nothing more. The open-ended faucet exists only when a loop lets her tap repeatedly. The card-draw clause is the gentler half of the design. It triggers whenever any other creature enters the battlefield, but the power comparison is checked on resolution, so the controller draws only if that creature outsizes everything else in play. That structure rewards a curve that keeps topping itself rather than one that goes wide. What makes her dangerous is the gap between her cost and what she enables: she resolves as an unremarkable green body but caps out at whatever the largest creature you control happens to be, not at anything printed on her. That asymmetry is why she reads as a value piece and plays as a combo piece.