Necropotence...
I know it's so huge that it's one of the two examples you gave and it's a broken card. I would have loved to have picked something else that's not quite as iconic or infamous. I like Pox, Charisma, Infernal Contract/Cruel Bargain, Kaervek's Spite, Doomsday, Seismic Assault, and, of course, Ball Lightning.
Actually, it really looks like all the coolest CCC cards are black. Yeah, I'd probably pick Kaervek's Spite. But I'm compelled to choose Necropotence. I was a loyal Necropotence player for a long time. I later shifted toward even more ridiculous cards like Yawgmoth's Bargain, Tolarian Academy, and Dream Halls. But Necropotence will always hold a special place for me because I played the thing so much and in so many different decks. If I had a nickel for every time I went Dark Ritual, Necropotence on the first turn, I'd have at least been able to buy some dual lands or something...