Hercules, Prince of Power

The lever here is the cost-reduction rider, which reprices the power-up depending on when you commit him. Leave him on the board as a plain 3/3 and the activation runs at full freight, a steep one-time toll for a single decisive pump. Cast him and swing the same turn, though, and the discount tied to his own casting leaves only to convert the whole package: a +1/+1 counter plus vigilance, indestructible, and haste, all at once. That inverts the usual bargain around haste-granting effects. Ordinarily you want a creature that has sat untapped and patient; this design pays you for the sprint instead, rewarding the aggressive line where you deploy him and immediately spend on the attack. The "activate each power-up ability only once" clause is the ceiling that stops him from swallowing every spare mana over a game: he converts exactly one activation's worth of resources into a bigger, harder body, so the discount is the sole variable you control. The card that emerges is a fair 3/3 that stays honest until the turn you elect to make him a hasty, indestructible attacker who keeps vigilance to hold the fort on the swing back. He is a demigod most dangerous in his opening breath rather than after he has been forgotten, and the discount is the whole decision: unleash him this turn, or wait for a window that will never come cheaper.