I don't know about anyone else but, if I ever need something along the lines of the tools in this thread, it is always RAINING.
With that thought in mind, the ARC AAA LE is that hands down winner. While my eyes dark adjust to use the Photon II's (I have many of them), the light output of the ARC AAA is much more useful due the spread, color and, quantity of light. Cost of operation is very low for an ARC LE as well as having good and cheap batteries avialble everywhere (including 7-11 and other roadside stops in wee hours of the morning.
http://store.yahoo.com/flashlight/buynow.html
For fire, get a swedish fire steel - specifically the smaller boy scout model since this is a keychain after all.
http://www.light-my-fire.com/firesteel/?CatId=2
A compass should not be under estimated. One with a signal mirror would be tops but, you must balance the size issue.
Get a good durable tough as nails WHISTLE that doesn't require the lungs of an Olympic athlete to produce ear splitting volume. If you are tired or hurt, a whilstle that is easy to blow could be the difference you need.
The small multi-tools and a swiss army knife should be considered together to avoid tool duplication and excessive weight and size. I like a Swiss army knife with a large straight blade and scissors. Scissors come in darn handy in motel rooms and office buildings when you need to trim a stray hair, thread from your slacks, finger nails, etc.
Now, how about some emergency money? This is a survival key chain after all. Food, telephone calls, etc. generally all require some cash when we make it back to civilization.
Some high tensile fishing line and a means to carry water (a balloon maybe) should be considered as well.
Hmmm.... survival keychain or, handy do-it all keychain....