Unlike a lot of apparel, bicycle helmets are one of those things you really have to try on in person to make sure it's a good fit. Over on The Sweethome, they break down what a good fit actually means using the easy to remember "low, level, snug" system.
Picture: New York City Department
As far as safety is concerned, all bike helmets are the same, so it's mostly about fit and style for most people (and weight, if you're racing). Here's how to check the fit:
Labels like "Universal Adult" can be misleading -- often these helmets are equal to medium models from other manufacturers, so they may only fit average heads. Choosing between S/M/L/XL will get you a better, safer fit.
...start thinking about how the helmet fits and secures itself to your head... repeat these three words when fitting your new skid lid on your head: low, level, snug. Your helmet should be situated in the middle of your forehead, a little bit above your eyebrows so it doesn't get in the way of your vision. Make sure your adjustable retention system (if your helmet has one) is snug enough that if you do your best head-banging Slayer fan impression, it won't budge more than a centimeter or so. Once you've done that, tighten your chin strap so it is snug but not choking you. Two fingers width between your chin and strap is typically the rule of thumb. Finally, adjust the strap yokes (if they're adjustable) so that the straps go on either side of your ear and don't lay on top of your ears.
As far as the best helmet for commuters is concerned, The Sweethome handed their pick over to the Specialised Echelon because it was comfortable and fit well on most riders. Still, it's always best to test a helmet out on your own head before you buy, so be sure to head into a local bike shop instead of ordering online. Of course, make sure you're wearing the thing the right way too.
The Best Bike Helmet for Commuters [The Sweethome]