For me, it's:
>Poncho
Serves as an adequate shelter, rain layer, pack cover, hammock, chair, and general hobo sack. Small enough to bring just about anywhere and I use it all the time.
>Heavy wool shirt jacket
Thick, lined wool shirt/jacket. Probably the warmest thing I own. I've never been cold in it.
>Headlamp
Self explanatory. I have a black diamond and a petzl, but I'm going to huy another princeton tec with a sliding red filter so I don't accidentally blind myself when I get up to piss
>Canteen cups that fit my nalgene
I have two that almost perfectly encase my nalgene so I can have two cups (one to cook with and one to eat out of, or one to boil water in, or whatever) that don't take up any extra space
>Evasion chart
Tyvek map with descriptions and pictures of local flora and fauna, land navigation, first aid, and other stuff. It's like 4'x5' and just having a big ass piece of tyvek is really nice to use as a clean spot.
>Knife/leatherman
I've had an Esee 4 for like 5 years and I love it. I have a leatherman wave that also gets a ton of use all the time.

But really my favorite things are my poncho and my wool shirt.