The Irish judge lavished praise on Marvin Humes, Oritse Willams, JB Gill and Aston Merrygold when he was asked to name his favourite act from across ten series of the show.
"To be honest, I always remember JLS in their first audition. When they walked in they were the perfect group. Not just vocally, but mentally and the way they worked together," he said.
"I loved working with a lot of other people – Shayne Ward, Jedward for different reasons. But the most perfect act for me was JLS."
Louis mentored JLS on the fifth series of X Factor, where they finished runner-up to Alexandra Burke. Despite not winning, the group went on to have massive success, selling more than ten million records worldwide before calling it quits last year.
(We're still a bit upset by that.)
As well as praising JLS, long-time judge Louis had complimentary words for former X Factor contestant and Xtra Factor presenter Olly Murs.
When asked to name a contestant who defines what X Factor is, he replied: "Olly Murs – he's a massive star now. He didn't win the show but he had the X Factor. He really had it and he has a great work ethic. Also Rebecca Ferguson, when her record comes on the radio you know it's her, and that's what the X Factor is all about."
X Factor returns to our screens for its 11th series on Saturday, 30 August on ITV1. Louis will return as a judge alongside Simon Cowell, Cheryl Fernandez-Versini and Spice Girl Mel B.
"This year is fantastic. This was meant to be," said Louis. "There's good chemistry between all of us. Simon and Cheryl are very funny always annoying each other. Mel is just honest. This is the perfect X Factor panel, it really is."
Watch the official X Factor 2014 trailer below:
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