I've tried (fluffy) synthetic knitting yarn and mason twine, the twine is much easier to handle and more reliable.
Some plants I was given had pieces of nylon stockings as wicks, they appear to work well too.

Karin

Walmart has mason twine. I've used it and used stockings but didn't care for them. I have a big cone of some kind of synthetic macrame cord that is woven 8-ply so it makes a lot of wicks.

I've been sold the mason twine at outrageous prices when I didn't realize what I was buying. I guess it costs a lot to cut off a piece :)

Diana

I prefer masons twine. A 500 ft roll is a few dollars at Lowe's, but make sure to get the synthetic one as they also sell some that are a blend of synthetic and natural fibers. I use 1 length for any pot less than 4", 2 strands for a 4" pot, and 3 strands for a 5" pot. I'm not sure that I really need extra strands in the 4 and 5" pots, but I like to err on the side of caution. I've tried acrylic yarn in the past and found that under my conditions those wicks failed more quickly than the mason twine wicks did. Because everyone's conditions are different, the only way to really know which will work best for you is to try a couple plants with each type of wicking material and see which lasts the longest for you. Be sure to experiment on only a couple plants at a time, and use plants that aren't near and dear to you heart.

Mark

I got my mason twine from dollarama. They even had funky colors to choose from ;-)
One per pot seems to be enough, but they appear to seize up if there is algae in the reservoir... Just have to rinse them every so often...

Karin

I did an experiment: 3 pots with dry wicking mix and wicks of mason twine, yarn, and polyester cording dropped in water-the pots w cording and yarn became wet quickly, and the mason twine took an additional 24-48 hrs to become wet

I like synthetic crochet thread. It's a smaller than #2 yarn, wets quickly and seems to deliver the right amount of water to a light mix. I'm slowly swapping out all my yarn wicks for the crochet thread. I still soak it in soapy water to remove the sizing agent, and then pull it through the drain holes in my pots.

--Byron.