The Bangor man starred for Ireland across all five rounds, showcasing his creativity and passing ability with six try assists.
McCloskey also ranked first for Turnovers won (8), Dominant Contacts (18) and Minutes Played (400).
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France's Louis Bielle-Biarrey, Italy's Tommaso Menoncello and Scotland's Kyle Steyn are also shortlisted.
Voting is now open and will close on Thursday 26 March at 0900 GMT.
You can cast your vote via the Six Nations website.