Sorry, can't pick just one. Current top five off the top of my head:
My regional transportation card. Right now I work in a town that is 40 minutes away by train, so recently bought a train/bus card that is unlimited use any time of day/night for a month in the county I live in, which is slightly larger than the US state of NJ. A lot of the E1 trails pass through this county, and I am in the process of hiking them, so it's a win-win situation for me with work and play. Also, it weighs like 4g.
Zpacks Arc Blast backpack. It's just such a great pack, and can handle anything from winter weekend trips to longer section hikes. If/when I do a thru-hike, this would be the pack I would take.
Waterproof/resistant maps. Recently bought both Tyvek and Pertex treated maps, that are also the most updated (2013). I love looking at maps. When I am stressed out or feeling down, often I will break out one of my newest maps and read it for a while, daydreaming about future trips. That I don't have to waterproof my maps myself saves a lot of time and weight, and is like the icing on the cake. I also love finding things that are not marked on maps that are nice little surprises. Last weekend I did a section hike and found three different shelters, all of which were really nice, and none where on the map.
Borah gear down vest. So light yet so warm, and such a versatile piece of clothing. Great at night and in the morning when its colder, and great to sleep in, plus fits nice under or over other layers. It's a great upgrade to my sleeping system--it's like I upgraded my 5C Zpacks bag to a 0C bag. Very comfy, simple, elegant, easy to pack, 105g.
Camera. I am no photographer and just have a point and shoot little digital camera I got like 4 years ago. But over the past few years I have taken more and more pictures and family and friends have said that my skills are improving and that I have a better eye for a good picture. It's also great to capture some of the beauty that one encounters, and helps one to remember details from trips and of the area. Looking to upgrade in the near future, but cameras in general have earned a new place in my heart as a backpacker. I used to be one of those "memory is best" type people and didn't want to be bothered with taking the time to snap a bunch of pictures. Now I enjoy it, and found it takes very little time and is really worth it.
Wow, picking just 5 was hard enough. I don't know how a lot of you just picked one! I think most of my gear has made me smile at one point or another.