Having owned a shop for 10 years and owned and personally tested a ton of helmets, the main thing to look for is what material the helmet is made of. Stay away from plastic or polycarbonate helmets. These are a helmet shell that requires a thicker EPS (the inner Styrofoam) to help absorb the impact. The shell it self doesn't absorb anything. Whatever the shell and EPS liner can't absorb is transferred to your brain since thats simply in "fluids" inside your head. Having a thicker EPS leads to a bigger helmet. Added weight and less aerodynamic.
The best helmets are a composite shell. Some combination of Carbon, Kevlar, fiberglass, and companies own personal blends of fibers. These shells are designed to spider-web upon impact and distributing the impact to a wider area of the EPS. The Shell absorbs the brunt of the impact with the rest transferred to the EPS liner, leaving far less to be transferred to your brain. This allows the shell and liner to be thinner and lighter.
A good way to think about a helmet in a real life situation is a wreck at 30 mph. With a cheaper helmet you'll be ok and walk away with a . Headache or concussion. With a high quality helmet. You wont feel a thing. No headache. And at higher speeds the gap widens. Its your head. Protect it. It wont grow back.