I agree with what hikermor said.

A reasonable level of light and long run time are probably the most important factors. Most lights now will let you change levels (low for battery life and high when you need lots of light). Other things to look for are robust construction and water resistance. I like to standardize on plain old AAA batteries, since it is easy to carry spares, share with others in your team, and you can find them at any gas station or convenience store on your way to the field.