Adamantium. Arguably the toughest metal in the Marvel universe. It has shrugged off Hulk punches and bludgeons from Thor's hammer. The question I pose however is, would Superman be able to break/dent it? He can after all, do [url=http://media.comicvine.com/uploads/6/67064/2081435-supermansuper_strength_super.jpg]this.[/url] (Not even his greatest strength feat)
I'd imagine it would be like one of those unstoppable forces meets and immovable object kind of thing.
EDIT: No sundips and no punches that involve flight or speed. Just a regular ass standing on the ground punch.
EDIT 2: Changed from Vibranium to Adamantium because Adamantium evidently seems more durable.
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1. Superman melts the Adamantium using the lasers from his eyes. 2. Superman pulls it apart while it is in its liquid state 3. Superman turns it back into its solid state using his breath. There, he broke it, kinda.