Top place has got to be my two walking poles, which I've used for a few years now. They really do take some of the strain off the legs. Before I had them, I used to finish my backpacking trips in Snowdonia hobbling around for the mext three days like a bandy-legged seaman on his first shore leave for months, but that is a thing of the past since I've had them.
Second place goes to anything made of fleece. I'm old enough to remember wearing all wool items, which were itchy to wear, retained moisture, took ages to dry, either shrank or stretched, and wore out around the hems and elbows. Fleece is lighter, seems to be indestructible, is easily freshened up in the washing machine and can even be worn straight afterwards if you want.
In third place I would nominate my Field&Trek rollup water carrier, which I have had for absolutely years and it is still as good as new. Inside the tough nylon outer is a bag similar to those in boxes of wine, and I believe you can get a replacement for this if needed. It has a clever rubber cap with a "tap" that you simply press with your thumb to operate, and in spite of its weight when filled it will quite happliy hang with a loop on a branch or stone wall.