The best fabric for digital printing would be 100% cotton as this lends itself to the process, but other fabrics work well. Much depends on the model of the printer as these do vary in what they can offer and produce.
The more budget-friendly printing machines can print well on cotton, cotton blends, linen, viscose and leather. Whereas high-spec printers are able to print on almost any type of fabric including silk, polyester, wool, denim and lycra to name a few.