Hmm, what is my favorite piece of gear, this will require some thought.
They're not big sexy items but what I find most useful, other than items previously mentioned such as headlamps, would probably come down to...
Tree hangers. I picked two of these up surplus years ago. They're simple nylon straps with a hook on one end and a load binder style latch. You sling them around the tree, snug the strap, then push the handle down and it's secure. It'll work with up to about an 18" tree, in a pinch I've combined the two to handle more girth.
I've never had a load slip on them, I typically use them to hold five gallons of water up in the air. I sure wish I'd bought more than two of them, they're that useful.
Now about that water...
Another favorite are my Swiss surplus five gallon / twenty liter water bladders. They're made out of thick rubber and have a push-button valve on one end. I have four of them although one of them is losing the coating inside so is not for potable use anymore, I'd love to find something this heavy duty and convenient. Typically I toss a tarp over it when camped so the sun doesn't warm it too much, even if I don't the water generally doesn't get terribly hot as it's thick rubber.
Sea2Summit collapsable kitchen sink. I have two of these, yeah they work for kitchen duty but I also like to keep them full and handy at night in case the campfire goes stupid, good for making sure that fire is dead out before departing in the morning.
And last for today is my Amazon Kindle with lighted book cover. Good for a read by day or in the middle of a moonless night in the woods.
I'll probably have more items for this list after I go backpacking this spring, it will be the first time in more years than I care to announce.