Cersei Lannister, as we all would like to see her end up.
Actually though, Feanor? Though can't say he's that "unsung" and infamous - his name is still honoured. But he did start preeety much the history of ME as we know it, and especially the First Age shenanigans. If he'd only listened and given the Silmarils over, and not followed Morgoth to Beleriand to get them back...