之 here is the classical possessive/attributive particle (= modern 的). 经典之作 literally parses as 经典 'classic' + 之 'of/'s' + 作 'work'. The 之 form sounds slightly more elevated than 经典的作品 — fitting for reviews, essays, and praise.
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Use 经典之作 for serious praise: novels, films, albums, paintings. Don't use it for trivial things — calling a meme 一个经典之作 sounds tongue-in-cheek (which can be intentional).
