You must prioritize (as always) with what you decides to carry. In some situations shelter is the most important thing, and you may need it fast, so cordage and an emergency blanket. Signal may be all that you need, so light, whistle and something in a bright color. In very hot environments water will be critical, even before you get into a survival situation.

If you plan properly (environment, routes, leave a note, etc), and are dressed appropriately, it can really reduce the amount of other stuff you have to carry. I always go back to it, but your brain is your best tool.