All those lights are pretty bright when the batteries are brand new, and dim considerably after just a few minutes. They stay usably bright for hours though. Do you only care about the first few minutes?
Also, what's your definition of a "keychain light"? Some people around here have Arc LS's on their keychains. There are also mods like the Microlux and Micro Illuminator.
The coin cell lights are dead simple, they're just coin cells direct driving an LED (exception: Photon III), and they all use pretty much the same cells and LED's. There shouldn't be any significant difference in brightness other than from the battery freshness and from random differences in the LED's.
If you want a good practical keychain light that will run for a long time, get an Arc AAA.
