Five lights stand out in my mind as the "most useful in an emergency" lights for me:
Jet-III Pro Ti & ST, Jet-I Pro EX, LF5XT, L2D
Oh, and an H50 if I could remember where I put the damn thing.
I'd use the Ti & ST until I ran out of 18650's, then fall back to the AA lights.
I got practise last night during a two hour power outage, and both the ST and EX really did prove to be quite useful, especially with an F04 beam diffuser.
Least useful: Anything RCR123/CR123A powered. 18650 beats them with capacity, and AA beats them for "can buy it anywhere" replacement cells, at least in Australia.
To be honest if I wasn't pressed for space I'd probably just grab my container of demo/tester lights, which has one of just about every Fenix, Nitecore, and JETBeam model made in the past year, all with fully charged batteries. It is small enough to not nitpick about wasted space in the car. The Pila IBC would of course come along, using the DC adapter.
Regards,
Eric