I really love the bamboo/cotton muslin I bought in Joann's last year. Enough so that I went and bought a second bolt.
Otherwise, I use high quality white sheets- thread count about 250. I know that many don't like to use sheets, but I buy sheets and sheet sets in Ross or TJMaxx and don't care if they are irregular. The first thing I do is rip off the elastic on the fitted, and tear it into rectangles more easily used.
I often use the fitted sheet (of any color) as the sashing in a quilt, then I use the flat sheet for a backing, and give the quilt with the pillowcases, which match exactly.