I popped out this morning and bought another two pairs of Decathlon 'Kalenji' boxer underwear. I'm a 32" waist and use their 'Small size' for running and their 'Medium size' for hiking. When I first bought my first couple of pairs they were £4.99 per pair, now they are £3.99 per pair.
When I hike I find them so very comfortable, they are made from an elasticated mesh type material and just figure hug in all of the right places. I just wash a pair each day (I wear them when I shower and wash them in that way, so easy) and they dry very quickly. Also, they come in a variety of colours so I never have to make a mistake of wearing the wrong ones.