The best yarn bowls have several important features. They are not all created equally, so it is wise to know beforehand what you will want to consider when searching for the perfect one.
First of all, the bowl will need to be large enough to accommodate the ball/s of yarn for the project comfortably. Some knitters work with a few skeins at a time and would benefit from a larger bowl with several holes through which they can guide their various colors. For knitters who like to knit on the go, a more compact, portable bowl might be the preferred size.
Also bear in mind that the yarn needs to feed through the grooves smoothly and without any tension or snagging. The potter (or maker) has had to have taken great care to ensure that all rough edges have been sanded off and the swirl tapers to its final route gradually.
There needs to be enough of an upward curve in the cut out to keep the yarn in place. With a straight cut, the yarn can easily pull up and the ball will be pulled from the pot. A "J" shape is the ideal for this purpose. And a simple line works better than a complex one.
A yarn bowl should also be pretty. Just as a skein of yarn is visually appealing and feels good in your hand, the bowl should be something worth looking at and should make you want to pick it up. The design of the bowl, from its overall shape to the gentle swirl with a clustering of small holes, should interest the eye. A yarn bowl's purpose is to house the softness of the yarn with grace and double the enjoyment of the handiwork process. Nestle the bowl in your arms to determine if it is right.
Knitting is a meditative work that focuses on tradition and love of the craft. The bowl that a knitter uses should have been made with that same intent.