My own experience has been that for a "serious situation" light there are several factors to consider.
Brightness - Generally you want to find something in the 80-100 lumen range.
Flood vs Throw - Generally you want a somewhat wider "flood" type of beam so it lights up a whole area at once not just a spotlight on a tiny space.
Physical size - a modest size means you are more likely to have the light with you when you need it.
On one end of the spectrum,
these Energizer flashlights (sold at Target) are really excellent for the money ($45 or so). I have one in each car as they are tough and the lithium batteries are not affected much by cold.
At the other end of the spectrum is pretty much
anything by SureFire (now being sold at Home Depot of all places). Often quite expensive but tough and then some!