I've used:
Cottonman cotton
Cotton bacon
Organic cotton balls from CVS
Japanese cotton
Between all of them there are upsides and downsides:
Cottonman is not readily available and I had to use two or three strands of yarn to fit the coil properly. This makes it a bit difficult to work with. That said, it is supr clean. Even after a few days of vaping it, it hardly looked burned at all. Flavor was superior to regular organic cotton balls and dry hits were not nearly as bad as anything else I've tried. It also wicks like crazy which may explain how it stays so clean.
Cotton bacon is a bit expensive for what it is. It seems to be just organic cotton delivered as strips instead of balls. It's easier to work with than cotton balls, since no unrolling is required, but it is more difficult to get uniform wicks. Flavor is on par with organic cotton balls, no better or worse. It is also on par with cotton balls in terms of cleanliness, and dry hits hurt just as bad.
Organic cotton balls are what they are. You can pick them up at just about any chain pharmacy store. Personally, I hate unrolling them which causes me to rank these the most difficult to work with of any that I've tried. In terms of cleanliness, I change my wicks about every two days (three if I'm not vaping heavily) and the use shows on the cotton. Flavor is clean, and is the baseline by which I judge other types of cotton. Dry hits hurt.
Japanese cotton is by far the easiest to work with. I steam mine and fluff it out a bit before use, this greatly improves it's (already good) wicking abilities. That said, it does not wick as well as cottonman, but does wick better than cotton balls or cotton bacon. Getting uniform coils is a breeze. I order mine from amazon, so a little forethought is necessary if you don't want to run out. Flavor is better than cottonman, cotton bacon, and cotton balls. Dry hits are less severe than with cotton bacon or cotton balls, but not as lite as they are with cottonman. In regards to cleanliness, I do change wicks every two days, and the use on the cotton shows.
This is all just my opinion, of course, but I wanted to share my thoughts on the pros and cons I've experienced with what I've used.