Dan Stevens, best known as aristocratic Matthew Crawley, won out over Twilight heartthrob Robert Pattinson (no 7), suave Bond star Daniel Craig (no 25) and football's fashionista David Beckham (no 36).
The Duke of Cambridge made it into the list at number 37, with GQ crediting wife Kate with "upping her husband's fashion game".
But it was William's father who ranked eighth - just one place behind R-Patz - prompting the magazine to note: "Despite sticking to a classic uniform, he always looks on trend."
HRH may be bemused at being a fashion icon, but he's proved himself the king of tailoring.
Singer/song-writer Ed Sheeran was named worst-dressed man, followed by Labour MP Jon Cruddas and reality star Alex Reid.
Olympics and Tour de France tour de force Sir Bradley Wiggins is among the new entries in the best-dressed list, featuring at number 30.
After Dan Stevens, the top five is rounded out by actor Tom Hiddleston, who counts War Horse among his acting credits; rapper Tinie Tempah; Radio 1 Breakfast Show host Nick Grimshaw; and X Factor presenter Dermot O'Leary.
The Best-Dressed Men List is in the February issue of GQ, available from Thursday.