My list (ordered randomly):

1. Sidearm
2. Spare magazine
3. Flashlight (generally I tote a Malkoff MDC)
4. Cell phone
5. Cash (my habit is to keep a couple hundred bucks)
6. Keys
7. Vehicle (so keys)
8. Spare battery
9. Extra clothing layer (in car)
10. Map of the area
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“I'd rather have questions that cannot be answered than answers that can't be questioned.” —Richard Feynman