There are several factors to consider when you’re trying to sort through the best headlamps.
Brightness
Headlamp brightness is typically measured in lumens. The higher the lumen count, the brighter the light.
Generally, anything over 300 lumens is sufficient for hiking or general tasks. You may want something brighter for trail running or if you want longer-distance illumination.
Some headlamps have 1000 lumens or higher, which is incredibly bright. These high-power headlamps are awesome, but they also use a lot more power and drain their batteries quickly on the highest mode.
Weather Resistance
If you’re using a headlamp for hiking, running, camping, or anything else outside, there’s a good chance it will get rained on.
Most headlamps are IPX4 rated, which means they are tested to withstand splashing or jets of water. This rating is completely sufficient for rain and weather protection.
If you want the absolute best water resistance possible, look for a light that is rated IP68 or IPX8. These are the best headlamps for severe weather because they are fully submersible and entirely waterproof.
Disposable or Rechargeable Batteries
It used to be that the standard was for headlamps to use three AAA batteries. That’s still the most common option, but now manufacturers like Black Diamond and others are building more headlamps with built-in rechargeable lithium-ion batteries.
Standard batteries are convenient, and it’s nice that you can find them anywhere, but they often end up costing more in the long run. Plus, if your batteries die and you don’t have any on hand, you’re SOL until you buy some more. AAA batteries are also much worse environmentally.
Rechargeable lights are more expensive up front, but I think the extra cost is always worth it, especially if you use your headlamp frequently.
Battery Life
Whether you choose a headlamp that uses AAAs or recharges with a USB cable, battery life is a major concern. The last thing you want is for your headlamp to die when you’re in the middle of a task or out on a hike.
Most manufacturers advertise run times on different brightnesses. I would always take these with a grain of salt, though. I used a headlamp that said it runs for five hours, but after two hours it was so dim that it wasn’t much use. I recommend always reading reviews to see other people’s real-world testing and experiences with battery life.