Breathable cotton, don't tend to "bunch up" at knee crease, good quality, light compression (I have "small knees/legs" and a size medium provides light compression). Can also dye them if you are tired of white (or they get stained/dirty and are no longer white!).
Finally found a knee sleeve that gives me just the right amount of light compression. Breathable, good design. Very little "bunching" at the back of the knee also. Also dyes well (white does not stay white for long, and it's summer weather, so I have dyed mine with an over-the-counter dye to match my outfits).