Demonfire

In Commander, Demonfire slots into storm and mana-doubler shells that can generate obscene amounts of mana in a single turn — the hellbent uncounterability clause matters most here, where interaction is dense and a telegraphed X-spell is otherwise easy to answer. In Modern and Legacy, it competes with Grapeshot and Tendrils of Agony for the storm kill slot and mostly loses that fight, since those cards win through multiple opponents or through blockers in ways Demonfire doesn't. Vintage gives it the same competitive landscape with even fewer reasons to pick it over established finishers. Demonfire is legal in Oathbreaker as well, where its ceiling is the same as Commander — high in the right shell, irrelevant outside it.