The actor topped the list of best dressed men in the UK, which also featured Bradley Wiggins.
Dan, whose character Matthew Crawley was killed off in a shock exit from the ITV1 costume drama at Christmas, triumphed over heart-throbs Robert Pattinson, Daniel Craig and David Beckham in GQ magazine's annual list.
The Duke of Cambridge is 37th best-dressed but his father is ranked ahead of him in eighth place, one place behind Twilight star R-Patz.
GQ magazine said William "epitomises true British style", adding: "Has the Duchess of Cambridge upped her husband's fashion game? It would appear so."
But the magazine says of Charles that "despite sticking to a classic uniform, he always looks on trend", adding: "This is a man who knows what he likes, knows how to wear it and sticks with it whatever the prevailing fashions."
It said: "HRH may be bemused at being a fashion icon, but he's proved himself the king of tailoring."
Newly-knighted Tour de France and Olympic cycling champion Sir Bradley Wiggins is a new entry in the list at 30. Other new names are Homeland star Damian Lewis (12), One Direction singer Zayn Malik (17), cross-dressing artist Grayson Perry (22), and Paralympic champion Jonnie Peacock (48).
After Dan, the top five is completed by actor Tom Hiddleston, rapper Tinie Tempah, Radio 1 Breakfast Show host Nick Grimshaw and X Factor presenter Dermot O'Leary. The Wire star Idris Elba is sixth, followed by Twilight star Pattinson and Charles, while funnyman David Walliams is ninth, with My Week With Marilyn actor Eddie Redmayne completing the top 10.
The top 50, compiled by a panel of experts, also includes Take That star Gary Barlow (11), Sir Elton John (21), Sherlock star Benedict Cumberbatch (24), Bond actor Daniel Craig (25), Prime Minister David Cameron (28), Jude Law (35) and David Beckham (36). The Best-Dressed Men List is in the February issue of GQ, available from Thursday.