it goes without saying....
buy the one you'll wear! Try on several and get the one that is most comfortable for you. If you are riding on the road I suggest a bright easy to be seen color and be sure to wear it snugly as per the instructions that come with it. There probably aren't any "perfect" helmets after all there probably aren't any "perfect" heads! LOL
Something to remember with any helmet if you fallen and hit your head your helmet even though it won't look damaged the protective structure of it has been compromised. Send it back to the company for a replacement, they will usually replace it for around $20, you pay shipping. I keep the original box just for this purpose and a back up helmet so I don't miss any riding time.
I've done this twice once Specialized and once with Giro. My experience with Giro was a very good one and I love their helmets for me the fit is a good one and the colors great.
Life is like a 10 speed bike, we all have gears we never use.
Charles Schultz
"The bicycle is just as good company as most husbands and, when it gets old and shabby, a woman can dispose of it and get a new one without shocking the entire community." — Ann Strong, Minneapolis Tribune, 1895