Maxine Rosenthal gives a lot of tips for fabric selection in her book, One Block Wonder, and shows most of the original fabrics for the quilts displayed in the book. I highly recommend it.
It isn't clear from your question whether you are stacking the fabric before making your blocks, but I'll assume you are. You want to find fabric without much "background", or where the background has movement to it. Avoid stripes, because they will produce straight lines in the kaleidoscopes. Larger scale prints are better. Florals are often used. I used a fabric with lots of different dog breeds in my first OBW, and it worked well, especially because people have fun looking for the dog parts.
If you're making a hexagon block, you need at least 6 repeats of the pattern, and for an octagon block you need 8. I find that makes it a little hard to experiment with fabrics in my stash. Good luck!