Honestly, a Big Fig would probably work better than just a standard minfigure with an added torso piece. We have a couple descriptions of his physical forms:
"...because of his mood and the malice that burned in him that form was dark and terrible. And he descended upon Arda in power and majesty greater than any other of the Valar, as a mountain that wades in the sea and has it head above the clouds and is clad in ice and crowned with smoke and fire; and the light of the eyes of Melkor was like a flame that withers with heat and pierces with deadly cold." [Silmarillion; Ainulindalë]
"Morgoth came, climbing slowly from his subterranean throne, and the rumour of his feet was like thunder underground. And he issued forth clad in black armour; and he stood before the King like a tower, iron-crowned, and his vast shield, sable unblazoned, cast a shadow over him like a stormcloud..."
I'd say that either a CCBS style figure, akin to Sauron's boss form from Lego Lord of the Rings, or a brick-built figure like this guy would work best.
