Are there any rights you think should supersede contracts? If so, how do you draw the line between rights that do and don’t?
(I’ll answer your question in a comment side-chain, just because you asked.)
Germans have the right to continued wage payments if they need to take care of family members (§616 BGB). However, that right can be voided in the employment contract.
(§618 BGB) essentially states that the work environment must be reasonably safe. This cannot be voided by contract, as is codified in (§619 BGB).
These are just instances. I do not know any general rules for the precedence of contracts over the law or vice versa.