Hulk doesn't really have one. Beyond that he's not invulnerable. He just regenerates really fast, and I'm fairly certain he can regrow lost limbs. But Hulk vs. Superman is a very contentious issue. As when Marvel and DC had their vs. series in the 90's they left several fights up to a vote. Naturally Superman won, and many feel Hulk was forced to job to Superman.
Besides the more interesting prize fights would be Superboy-Prime vs. Sentry
Or Superman vs. the Supernatural version of Michael.
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