Submit Feedback

To just make a report, the easiest way is Apple’s  Feedback  site, but it’s pretty general, and hard to provide the kind of detail that Apple’s engineers can use to "home in" on the problem so they can reproduce and fix it.

That’s also the place for suggestions and enhancement requests.

But you won’t ever get a response, so it’s hard to tell if/when something is really wrong, much less whether it will ever get fixed.

Contact AppleCare (Support)

If that isn't possible or convenient, or doesn't apply, usually, the best way is to call AppleCare: 

1. U.S. iPod, Mac and iPad technical support: (800) APL-CARE (800-275-2273). 
   

2. Elsewhere, see:  Contacting Apple for Support and Service
   

If your Mac is less than 90 days old, or you have the optional 3-year AppleCare coverage, the call is free.  Otherwise you’ll be charged (about $50 in the U.S.).

The AppleCare folks can usually determine fairly quickly if it’s an already-reported problem, and if there’s a workaround, supply it, or an alternative.

If not, they’ll assign a Case Number, and work with you to diagnose it.  They may walk you through various procedures, and perhaps e-mail you a "Capture Data" app to extract pertinent information from your Mac that you can e-mail back.

They may do some research and call or e-mail back for more information, or to report the results of their testing.  If they determine that it is a problem, and they can reproduce it, they’ll consult and follow-up with the appropriate Apple software engineers, and escalate to higher levels if necessary.

Unfortunately, though, they won’t tell you how, when, or if it’s going to be fixed, or notify you when it is.

The Genius Bar at an Apple Store

If you're near an Apple Store or Apple Authorized Service Provider, and can take your Mac to it, that's usually best.  There's no charge unless they have to replace parts or keep it for diagnosis (and of course, if it's within the 1-year warranty or you bought the optional 3-year AppleCare coverage, there's no charge for repairs unless they're caused by damage).

If the problem involves a peripheral, such as an external HD, Time Capsule, etc., you'll need to take it along, too.

Use this link to make an appointment at the Genius Bar:  http://www.apple.com/retail/geniusbar/

Or this one to find an Apple Authorized Service Provider: https://locate.apple.com/country  Select your country, then click the Service box.

File a detailed Bug Report

If you're a registered OSX developer, you can file bug reports in the normal fashion there.

You can register for free, then sign in at the Bug Reporter site.

It's fairly easy to use, with good Help. 

Provide as much detail as possible, of course.

You may get a request for more info, clarification, or they may close it quickly, saying they're already working on a similar report.  But you may not hear anything for weeks or months, either.  It all depends on how serious they think it is.