Longest losing streak: Curt Hawkins ~269 matches

The last time Curt Hawkins scored a victory was on November 8th, 2016, which was a fateful day for more than one reason. Since then, Hawkins has scored one draw against Dean Ambrose, but has lost every other match, to the point that it had now become part of his gimmick. He's basically the reverse Asuka.
Hawkins' fortune changed when he teamed up with Zack Ryder to defeat The Revival at Wrestlemania 35 for the Raw Tag Team Championship titles. It was a massive achievement considering how Hawkins' career was heading. The match was unfortunately on the Pre-show of Wrestelamania and it managed to draw lesser eyeballs than expected.
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Hawkins didn't get to keep his title for long enough as he dropped it back to The Revival and was soon released from the company along with tag team partner Zack Ryder owing to budget cuts. He is a part of Impact wrestling now.
Either way, this is a record that's unlikely to be surpassed by any superstar in the future, even the lowest rung of jobbers.