Tencel, rayon and linen are probably the fabrics which feel coolest to the touch.
You could make a wholecloth quilt of rayon challis (lots of beautiful, bright prints and very smooth) or buy a set of Tencel sheets and use the sheets as backing fabric for a rayon top.
You can buy Tencel fabric by the yard, but I don't know any stores which carry it. The sheets will be very fine and smooth - easy on the skin.
If you use a bamboo batting, it will wick moisture away from the body, and feel cooler, too.
I hope this was some help.