The TV presenter won Strictly in 2014, becoming the first contestant ever to achieve a perfect score for every dance in the final with partner Pasha Kovalev.
Clark, who hosts spin-off show It Takes Two with ex Strictly pro Janette Manrara, recalled former Love Island presenter Flack's dances as his favourite ever on the programme.
Asked who his stand-out performer was from the BBC One show's history, he said: "I always think back to Caroline Flack. It was so beautiful, and even more beautiful looking back now. It’ll always stick in my head."
Flack took her own life in February 2020, aged 40.
Clark and Manrara both agreed that one of their favourite routines was the silent dance from 2021 winners Rose Ayling-Ellis and Giovanni Pernice, choreographed to show her experiences of dancing as a deaf person.
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He said: "Rose and Gio doing ‘Symphony’ was amazing. When the music stops it was absolutely beautiful."
Manrara agreed: "The moment when Rose and Giovanni did their contemporary dance just showed the power of dance and the power of Strictly. It was so moving.
"And then, of course, watching my husband (pro dancer Aljaz Skorjanec) and Sara Davies, who is now a friend for life, dancing together, was wonderful for me.
"I love watching Aljaž dance, and Sara was a gorgeous partner and beautiful to watch on the dance floor too."
This year, Strictly has added a new theme week to celebrate the BBC's centenary which will see couples dancing routines inspired by the broadcaster's most famous programmes over the last 100 years.
Asked which BBC show he would love to be on, Clark replied: "EastEnders, as a long lost Slater."
Strictly: It Takes Two will return with behind-the-scenes chat and analysis of Strictly from Monday, 26 September at 6.30pm on BBC Two.
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