depends on where and how i am traveling. if i am driving across the country (i do this quite frequently) i at a minimum bring a case of water, jerky, phone and a sleeping bag.
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"the path of greatest resistance reaps the greatest rewards"
-me
For me the #1 is my phone. GPS, music player, ebook reader, video player and games all in one. Covers me from airport, plane, drive to the hotel and everything in between. The other two things are not as important, but my North Face technical jacket and collections of cables and converters.
My Jeep, as I don't care to travel by many other methods. Camera, as I like to document where I go and take pictures of beautiful things (plus my latest camera geo-tags the photos which is really cool for the location). And last, my Glock.