A Zafu is simply a circular meditation cushion that helps the meditator maintain a good meditative posture. But there are many zafus available, what makes a good one?
First, the zafu must help the meditator maintain a good meditation posture. While there are lots of meditation postures out there, the essential part is that the meditator should be able to sit calmly and relaxed in an upright position. Essentially a zafu needs to be able to create a good foundation to sit on and create a slight forward tilt to the pelvis so that the spine can remain upright with virtually no effort required from the muscles in the back.
The second aspect is the zafu should be built well enough to give many years of service to your meditation practice. This includes the stitching of the fabric, but perhaps more importantly the different materials themselves. The outer covering should be sturdy, comfortable and breathable.
The most important part of a zafu is of course the filling inside the cushion. Too soft and the zafu will lack the density necessary to provide adequate support for your meditation posture. Too hard and it will be so uncomfortable you won't want to use it!
I've used many different cushions over the years and very few seem to "cut it". Cotton fiber are perhaps the most common meditation cushion filling. It's not a bad filling, at least initially, but it quickly compresses in to a solid brick that doesn't provide much in the way of cushioning or comfort. It's rather dense as well and makes for a relatively heavy meditation cushion (which really only matters if you're going to take it on retreat or to a meditation group or something). Kapok is a similar fiber to cotton that is often used in filling meditation cushions. It has the added problem though of being highly irritating to some people's skin (and to the lungs and throat if the fibers ever get airborne).
Some folks will fill there "cushions" with either a dried bean or grain. While either of these will shape to your posterior, I find in using them that pretty quickly my bum has gone numb, and I want to get up from my meditation! The "hard truth" is that they don't provide any cushioning at all and most folks will find them uncomfortable to sit on.
buckwheat hulls are a comfortable filling for a zafu, but the problem is they breakdown very quickly. Within a year of regular meditations a buckwheat hull-filled cushion will start to feel deflated and soon be unusable.
What we've found to be the best filling for our zafus is wool batting. It is comfortable and supportive, breathable and resilient. It doesn't pinch off the circulation like some fillings will, and it being "breathable" means there's a little more air for your derrière so things don't get too moist down there. Finally it maintains its cushioning ability for a good decade or two.
Last, but not least, it's a real win if it's eco friendly. Organic cotton covering, eco-wool filling that isn't processed with the typical harsh chemical process - you get the idea. Save the planet and all that.
Everyone is different though and may have different preferences, so the best way to tell what will work for you is to try it! One thing to keep in mind if you're trying different zafus is that it is best if both "sit bones" should be on the cushion (rather than hanging off the front) to give proper support for your meditation posture. One strange fact I learned recently is that even with all the different shapes and sizes of humans on this Earth, there is very little variation in the distance between our "sit bones". Weird right?
And that's where I'll awkwardly end this. :)