The classic look in fashion (as well as in anything else), is all about proportion and harmony. All components of an outfit should be synchronized in a way that makes pleasant sense to the eye. The cut and color of the shirt should integrate well with the cut and color of the suit. Both the tie and shoes should complement the outfit, that can be completed with details such as a pocket handkerchief and jewelry, i.e. a watch and cufflinks, which embellish the overall look. None of these elements should ever clash in ways that are out of synch with the rest of the outfit, but instead contribute to an overall appealing presence.
The goal is to stand out subtly rather than with a shout, so that you do not stick out like a sore thumb, but instead produce a pleasant reaction in the beholder when you are noticed. This is in contrast to a strained effort to be noticed. That is easy enough to accomplish with overly flashy, loud or inappropriate dress if your goal is to amuse people or otherwise induce a negative response. The right way to be noticed comes from avoiding trying to be noticed. Ideally it results from something rare but attainable with experience, namely an effortless grace in comporting oneself. What I mean by effortless grace is what you see when Fred Astaire dances, hear when Heifetz plays the violin, or experience when an Olympic athlete excels beyond all expectations in a golden moment of flawless perfection.
In keeping with the purpose of the site the surest way to achieve this kind of impression is through a classic look. Historically the classical ideal was expressed in an elegant symmetry of elements comprising the whole of an object. These were proportionately balanced and harmoniously integrated to produce exquisite form never surpassed, echoing down through the ages. Thus the classic look is also something that has stood the test of time. It is not something that will be fashionable today and out of style tomorrow, but will transcend the vicissitudes of taste (or lack of it). Obviously clothing is not in the same category as say architecture in substance or historical time, but can still have a long life cycle. So for our purposes here a classic is something that will last as long as the garment is wearable.
A classic look is also the most time-tested way to appeal to the opposite sex, not on a visceral level so much as to enhance that by indicating that you are a man with some class. The guy with the flash or trendy outfit that conforms to what everyone else is wearing may think he’s appealing to women, but he's not.
There's a reason a guy dressed like Clark Gable gets the girl, apart from his natural appeal.