Q: Which of the six grounds for processing is the most important? Which takes precedent?
A: The simple answer is: the most important of the six bases for processing is the one that applies to the scenario in question.
It is not accurate to say that (for example), Consent is the “most important” grounds for processing and that therefore you should always seek consent first.
If your process has a legal basis, then you can process. It’s as simple as that.
Article 6(1), GDPR
No one basis for processing is considered “more” valid than any other.
You only need to establish one lawful basis for processing in order to carry out that process.
However, it is also true that, in certain circumstances, where two processes are in conflict, one may take precedent over the other.
For example, if data is being processed by consent, and that data becomes part of a legal case, the legal necessity will allow that data to be processed, even if the consent is subsequently withdrawn.