If the data you’re fetching is only a single or at most a few instances, then the Transients API should do the job; keep in mind it also handles the auto-expiration for you.
If the data could potentially be an unlimited (or at least medium-large) number of instances, especially if there may be even slightly complex ways of finding the correct entry to retrieve (besides just an ID), then a custom table would be advisable.
Having said that, there’s a third potential option: the file system. You could store the data serialized/jsonified in a file in a subfolder of wp-content. This is of course provided the data in question is effectively public and doesn’t need to be protected in the slightest.