AAAC had
a decent thread about this recently. Cuffdaddy is right about the missed opportunity of The King's Speech: so-so fitting center-vented suits trying to pass as Savile Row city suits. On balance, I'd have to say Redford or Waterson in The Great Gatsby. Brosnan as Thomas Crown merits mention:

His three-piece fits much better than Bale's in the Dark Knight. Dennis Price in Kind Hearts and Coronets wore some of the best clothes ever filmed:
Check out Sator's superb post on the film, now brought to a broader public by clothesonfilm.com.
Warren Beatty in Mike Nichols's 1975 The Fortune is up there, just for this suit:

TV, not movies, but Sam Neill as Sidney Reilly wore a fascinating gray three-piece lounge suit with a lapelless 10x5 DB vest. I think the coat was one of those Edwardian 4-buttons, too! Can't find an image. For me, the best clothes-in-wear ever filmed are on Anthony Andrews and Jeremy Irons as Sebastian Flyte and Charles Ryder in Brideshead Revisited. Check out
this photo. Throughout the immense series, they both always look completely natural in their costumes, just as much as Connery as Bond, who apparently slept in the Bond suits to prepare for the role. The linen or hopsack suit Sebastian wears in Venice, in particular, is spectacular. You have to see it in motion to appreciate it. It is the apex of easygoing elegance.