The Fenix E01 is outstanding, especially at its price. It has great runtime and is incredibly robust and reliable. I have one that I use for getting dressed in the morning while my wife sleeps, and several other secreted with various bits of gear. A number of people, however, don't care for its purplish tint.

I don't have a Fenix LD01 but by all reports its a fantastic light. While it doesn't have the runtime on low that you get out of an E01, there's something to be said for being able to produce 80 (emitter) lumens on a keychain light. It's also priced right for its performance.

The Quark AA is another outstanding light. I own an embarrassingly large number of Quarks, and my primary EDC flashlight is a Quark 123, sitting in my left front pocket. The only real downside of the Quarks is that they're too big for keychain carry, for me.

My keychain light of choice is the 4Sevens Preon ReVO, which is sadly currently out of production. It's small, light, has very long runtime on low and outstanding output on high (82 OTF lumens). I don't know any 1xAAA flashlight that performs as well.

I have no affiliation with any companies mentioned other than being a raging flashaholic.