You have the choice (Sorted by brightness (dimmest to brightest) and battery life (highest to lowest))! It depends of your needs. I own the Arc AAA-P (on keychain) and the L0D-CE (in pocket) and my girlfriend has a E0 on her keychain.
I have an LOD Holiday Edition on my keyring, I use the little black string that game with a mag solitare (makes it easy to manage). It's a great key light, bright when I need it but a low setting for hours of use should the need ever arise.
Also consider a LiteFlux LF 2 which can be either a Cree or SSC P4 LED. Peak makes a nice selection of AAA lights in several power, multi LED and body styles with body materials such as stainless steel, Alum, and brass.
I've had lots. They all have their plusses and minuses. Right now mine is the new Jetbeam. It's a well made little guy with three useful levels (4 if you consider strobe useful). Best part is it can use any battery. It looks to be pretty tuff - time will tell how well it stands up to keychain/pocket EDC duties.
The Millermods ARC Cree mod is my other EDC (in a cell phone pouch), but it's pricey, and the waiting time to acquire is difficult for a flashaholic to put up with.
Lots of choices. The basic decision you've got to make is whether you want a light that's almost indestructible but fairly dim, or one that's perhaps not quite so indestructible, but a lot brighter with a choice of different levels. The Arc AAA and the Peak Matterhorn with their potted heads are almost indestructible (and probably tougher than the Fenix E0), the Fenix LOD CE is perhaps the best of the current crop of multi-level high-tech AAA lights.
I personally EDC the Fenix LOD CE as my main "small" light, but when I'm travelling I've also got one of my Peak Matterhorns in my bag.
Another vote for the LF2. On sale now for $45 with a cree or ssc emitter. If you are looking for the least weight use a lithium primary in whatever you choose.
All those mentioned are good lights in their own ways. If small size and long runtime are most important go with the Fenix E0 or Arc-P. If extra power at a slightly larger size is needed go with the others.
Of all the lights I've reviewed (500+), I settled on the Fenix L0D-CE. It sits on my keychain and is always with me as my permanent EDC. How's that for an endorsement? However, if you can't stand PWM, you'd be better off with some other keychain light, since dimming is accomplished with a PWM circuit. Doesn't bother me at all for the short uses it sees.
I do believe the E0 does have the least weight. 0.8 oz with alkaline cell.
There seem to be reports of possible bugs, and the circuitry might not be quite worked out yet, but...
It looks like it might basically be the equivalent of an Arc-AAA with a Seoul emitter and multiple modes in it. :twothumbs
I've ordered one.
I had an Arc-AAAp and LOVED it. With all the new brighter and more efficient emitters, though, I was totally unable to justify its existance and sold it. In some ways I've been sorry ever since. I would love to have an Arc-AAAcree, and maybe we will someday.
Till then, this looks pretty close. I've got high hopes for this Jetbeam.