Price limit?
What I would suggest is finding a light with multiple output modes. Even a small light, if bright enough, can replace almost any flashlight you might want to carry. And the ability to use a low mode is amazingly useful. A bright level will suck batteries, but 90% of the time you'd only need bright for a brief period (if you need it longer, then you
should be carrying a larger light). With the low mode you can read maps, find the keyhole on your doorknob, and look inside your knife without blinding yourself.
My new favorite light is the Jil CR2 Intelli. It's a regular direct drive Jil modified (at the factory) with a circuit board that allows you to select 5 different user modes giving you different choices with brightness levels and strobes. For example, I use mode 4 which gives me low/high/burst (really bright, but only for short time use) plus a strobe, and SOS. The low/high levels are adjustable between 20 levels. Plus it can run on rechargable lithium ions. The beam is wide spot with a nice flood. Throws well considering the size. $105, but one might be hard to get ahold of at the moment (it's between revisions).
There's a couple other Jils available, the DD and 1.3W. The DD (direct drive) isn't as bright as the 1.3W, but it lasts forever. Several hours with a RCR2 li-ion (get the TWOJ version) or 20+ hours with a lithium CR2 (TWOJ). The 1.3W is brighter, and regulated, but only for about 1:10 before it starts to dim rapidly. It's larger than the DD/Intelli too. IMO, the only reason to get the 1.3W over the Intelli is price. A TWOH DD would be a great, not-terribly-expensive replacement for your Arc, since output is similar, but battery life is far longer.
The CR2 Ion that GarageBoy mentioned is a great light as well, but different. While the Jil is powerful enough to replace most larger single-cell lights you might carry, the Ion is specifically a flood light. Since I carry a larger light that can throw, I really like the flood of the Ion as a keychain light. Since it's a flood, the beam will be similar to your Arc, but brighter and whiter. Plus it has a low mode. Not a cheap light though, even at the base black HAIII price.
You could try the
MillerMods Fenix L0P. He takes a stock L0P and turns it into a 3 stage. He also can turn an Arc into a 2 stage Luxeon I (look at candlepowerforums for more info).
IMO 123A sized lights are too big for a keychain, but maybe not for you. If not, look at the DSpeck Fire-Fly III. It has the same circuit as the Jil Intelli, but it's driven harder so it's brighter (advantage of a more powerful battery). The FF3 is really a jewel of a light. Fantastically manufacturered and a joy to operate. The amount of light that comes out on burst is amazing for so small a light. But it's really more a pocket/lanyard light than a keychain light.
The Orb Raw, especially the new NS (which is even smaller), is the smallest CR2 powered light available. The "raw" part comes from the fact that it's just bare aluminum, so it'll scratch easily. Most Raw owners don't carry it on their keychain. The new NS (nickel silver) is different though, since the material is much stronger. Unfortunately though it's between production runs (I'm definitely getting one next time around). These have 2 stages. Plus you can get them decked out with tritium (like on Luminox watches), which glow brightly in the dark for several years (instead of a few minutes), so you can easily find your flashlight (and the keychain it's attatched to) in the dark. Plus, it looks very slick and high-tech.
Hope this gives you some ideas. Here's a couple pictures with some of the lights I mentioned, so you can get an idea on size comparisons.
Left-right: Jil Intelli (DD) | Fenix L0P | Amilite Neo T3 (Passaround) | McLux III PD (my EDC) | Fenix L1P | | DSpeck Fire-Fly III | Arc AAA-P | CR2 Ion (my keychain EDC)
