To look good, a suit must fit. And fit is the wearer's responsibility because he can't count on anyone else to do it for him these days. Most bespoke tailors won't let a customer out the door with a garment that isn't a great reference but I've seen some poor work from at least one great name recently. And ready to wear is let the buyer beware.
Fit has little to do with silhouette, fashion or construction, though all of these have a great deal to do with how well a man looks in his clothes. By fit, I mean the criteria that a jacket and trousers should meet to look as though they were made for the wearer and not his uncle or older brother.
First, the jacket must be long enough to cover the seat, but not so long that the legs seem to be shorter than the torso (another writer submitted recently that a man's fingertips should be able to curl under the jacket's bottom when his hands are hanging at his sides but that must have been written by a guy who never saw someone with short arms).
The jacket's shoulders should not be wider than the wearer's shoulders, and the jacket collar must sit snugly where the nape of the neck meets the upper back, staying in place through a reasonable range of motion. It should also lie flat behind the neck and across the collarbones in front.
The jacket should button without strain and there should be no more than three inches of space between the button and the chest.
Jacket lapels should fall straight down the chest without buckling or pulling away from the chest in any other way and the jacket back should not have horizontal creases anywhere along its length. If a coat does buckle or crease it is almost certainly too small, and no amount of alteration can make a RTW coat larger.
Jacket sleeves should not be creased where they cover the upper arm; creases here indicate that the sleeve is too tight or the shoulder has not been extended far enough. The sleeves should extend to the wristbone, allowing for a quarter to half an inch (depending on personal preference) of shirt cuff to show beneath.
Trousers should fall flat in front with no wrinkles, pulling, or creases over the stomach, and no tugging in the crotch. The pleats on pleated trousers, and suit trousers should be pleated unless a man buys them in an atelier in Naples, must be parallel with the trouser crease and must not pull open. Trousers should fall in a straight line, without ripples, to the shoe, where plain or cuffed trouser bottoms should touch the instep in front and just cover the back (some variation here is permitted as a matter of style rather than fit).