your going to get quite afew different responses on this. you can go with flint sand, pool filter sand, play lot sand, 3m quartz, tahiti moon sand, and so on and so forth.
I personally use pool filter sand, I used it cause it is cheap and very ray safe. doesnt look the best, but it serves its purpose. I have wanted to change it to something that looks better, but may do that when I change my setup.