I don’t wear a helmet currently, but I have tried out some here and there and have done a bit of research on them.
The folks at BHSI, a longtime online helmet advocacy group, claim the safest helmet is white or brightly colored (to make you easy to see), round (so there’s not something that can snag in a crash), no visor that can shatter into sharp edges, few vents that objects can penetrate. In other words your basic dorky mushroom helmet from the early 90s. I don’t think there’s been any realy studies to back up their claims, and a helmet that people will wear is safer than one they refuse to because it looks dorky.
Color doesn’t make a significant difference as far as cooling. A very old study was actually done that found that at any significant speeds there was enough air movement to prevent heat from building up in a dark helmet as opposed to a light one.
Bell and Giro are the biggest names, and are actually the same company but have very different fits, with Bell working on rounder heads and Giro for more squared of heads.
Basically you have:
Time Trial helmets- have those huge “teardrop” pieces in back. Only really for professional racers where every last bit of aerodymic streamlining might save a fraction of a second
Mountain and Road helmets- the standard ones you typically see and are best for most people. Sometimes the only difference between a “mountain” and “road” helmet is whether it has a visor or not, if you’re riding a road bike with drop handlebars a visor can obstruct your vision. Most visors are removable if you’re riding your road bike instead of a mountain bike that day or if it’s cloudy out, but my thinking is they could eventually break, the retention mechanism being hard plastic rather than metal. More expensive designs might have more vents in the front for a road helmet (move as much air while you’re riding fast) or on the top for a mountain helmet (let out the heat building up while you’re slowly climbing a hill). Some mountain bike helmets also have more rear coverage.
Skate helmets: Very round and shiny with a few, small vents, the Bell Faction and most of the Pro-tec models are examples. These have limited multi-impact protection and more rear coverage (you have minor slip-ups a lot while skateboarding), as well as one shot protection for higher speed bicycle crashes. Some people like the style, so skate-style helmets for bicycling only do exis, but when used for bicycling they’re a lot hotter than road or mountain bike helmets, with the extra rear coverage but with no visors.
BMX helmets- Like skate helmets designed for multiple impacts and hot for ordinary cycling. Some of them have chin protection more like motorcycle helmets.