Sorin, Grim Nemesis
Six loyalty at six mana is the floor of the pitch; the +1 is where the design pays for itself. Revealing the top card and taking it into hand is straight card advantage, and each opponent loses life equal to that card's mana value, so even a cheap flip nets a card while a fat one lands a four-or-five-point drain on the way to your hand. Self-refilling card flow is rare on a planeswalker that also plays defense: the -X trades loyalty for damage to a creature or planeswalker and gains you that much life back, which lets a Sorin played behind on the race buy back a turn it could not otherwise afford. The catch the loyalty enforces is that the removal is only as big as Sorin's current count, and you get one activation per turn, so the card can never stabilize and pull ahead in the same breath. Each turn is a single deliberate choice between digging for the lead and shrinking the board to stay alive, and that decision is the whole engine. The -9 manufactures a wall of lifelinking Vampire Knights equal to the highest life total in the game (often Sorin's own controller after a few ticks of drain), though a Sorin left alone that long has usually buried the opponent long before the ultimate fires. The body resolves a standing tension in Orzhov grind: the color pair wants to draw, drain, and trade one-for-one, and rarely gets all three from one permanent. This rations them across its loyalty, asking for no special support to do it.
