With all the flashlight scams out there, and with so many hundreds of models of flashlights using dozens of different battery types, it's very difficult for the non-enthusiast to wade through it all and choose the right flashlight.
There are so many good flashlights out there, a decent buying guide would be huge. Different people have different needs, so what I am going to do is break these buying guides down by battery chemistry. Your grandma probably isn't going to be using high-powered and potentially dangerous lithium-ion cells, and a police officer or fireman probably isn't going to carry a 1xAAA flashlight as a duty light. Different people have different needs.
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All of the lights in this post can use either rechargeable NiMH or disposable alkaline cells. Most people who know anything about batteries recommend "low self discharge" rechargeable cells like the Eneloop, which can be recharged about 1,000 times and can hold roughly 80% of its charge for 5 years.
Not only are alkalines a waste of money and bad for the environment, if exposed to swings in temperature while inside your device, they will leak acid, usually destroying whatever unfortunate device you put them in. That's why some people call them "alkaleaks".
In general the AAA battery is not that great of a power source, but it's extremely compact, and modern flashlights using it are very efficient.
1xAAA Form Factor
Flashlights using 1 AAA alkaline or rechargeable battery have come a long way in the last couple years, and are fairly incredible compared to the 1xAAA Maglight Solitaire I had in the 1980s.
Products in this space usually put out anywhere from about 30 to 120 lumens. Many people focus purely on output and miss the big picture. Most of the lights that I recommend have multiple modes where you can have high output for a short period of time or low output for a long period of time.
Having the ability to walk 12 hours in the dark using a flashlight the size of your thumb has definite survival implications. Some people prefer a single mode, high output light, so I'm trying to hit a wider cross section of different models.
The Lumintop Tool is the current high end. This model gives a good balance of features and a good selection of models, from the budget aluminum model to the titanium alloy one with the warm tinted Nichia LED.
Fenix is a well established and respected brand known for quality and reliability. Some people think they are over-priced. The LD02 uses TIR optics like the really high end lights and has the high-medium-low mode order that most people seem to prefer.
Thrunite has always been one of my favorite brands, because they usually put in my beloved moonlight mode, which is great for checking on sleeping children or using the bathroom late at night without waking other people up. And like Lumintop, models like this Ti5 cater to enthusiasts by offering models with a warmer, more pleasing tint, rather than the harsh cold blue tints of most LED flashlights.
Just a simple flashlight with one mode and a simple, "momentary on" tail switch. It's not the highest output or most technologically advanced, but it has a really good reputation for being tough and reliable.
2xAAA Form Factor
Modern LED flashlights using 2 AAA batteries can give you output that's in the ballpark of high-powered lithium-ion driven flashlights, while still using common cells you can find anywhere. Lights in this form factor generally start where the 1xAAA models left off and can go above 200 lumens.
But keep in mind that as incredible as they are, these two-celled AAA flashlights still must obey the laws of physics, and a tiny, high powered flashlight can't dissipate heat very well. So you won't find many products in this space that really push what two cells can do, but that's OK too, because these lights give you much better run time.
With good output, mode spacing and beautiful warmish tint, the Thrunite Ti4 is what I currently carry most of the time. It even has a superb pocket clip.
I've given a bunch of these Hugsby XP-2 models away as gifts and everyone loves them. They feature one mode with a simple tail switch, decent clip and very reasonably priced.
Another good performer and well regarded is the Thorfire PF04. Good price, features, modes, mode spacing, build quality--it's all there.