Try on a few and get one that fits your head shape best.
And doesn't make you look like a mushroom.
I like Limars. They sit a little lower on my head than Giros and have an adjustment knob.
+1 on Limars.. Giro used to fit me the best for many years but about 5 years ago they changed there sizing.. Found Limars at my LBS and have never used anything but Limars since.. I have the Pro 104 and 911 helmet.. The 104 is still one of the lightest on the market.