Use the year that belongs to the specific version you are citing, and record the other date in a note if your style requires it. The online-first date and the issue date describe different versions of the same article, so the first step is to decide which version your citation points to.
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Match the DOI and version. Look up the article's DOI and confirm that the record you are reading describes the version you actually used. A DOI resolves through the Handle System to current information supplied for the identified object, commonly a landing page maintained by the registrant, so the landing page shows what the registrant currently associates with that DOI. If you read the online-first version, cite that version; if you read the final issue version, cite the issue version.
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Inspect the deposited publication-date fields. A DOI lookup through the Crossref REST API returns the metadata deposited for that DOI. Check which date fields are present and what each one labels, because the online-first date and the issue date may appear as separate fields. Note any difference between the landing page and the deposited metadata instead of assuming one is wrong; treat a mismatch as unresolved and verify against the version record you used.
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Apply the required style to that version. Citing Medicine provides structured rules and examples for citing journal articles and other publication types. Follow your target style's rule for the version you selected, and use the corresponding year in the reference entry.
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Record the other date in a note. If the style permits or your instructor requires it, add a brief note explaining the metadata difference, for example that the article appeared online in one year and in an issue in another. This keeps the reference defensible if a reader sees a different year on the landing page.
You end with one year tied to a named version and a short note explaining any difference between the online-first and issue dates.