What's the best ratio for sand-only concrete?
First I want to point out that the hardness has a lot to do with cure time and temperature. Longer cures (6-8 weeks) at cool temperature (say 50 °F) are harder. Short, hot cures are harder initially, and crumble later.

Generally, the 1pt cement : 2pt sand ratio is best and will have a 3500 psi compression stress. A 1:3 ratio will have less than 3000 psi. Gravel can actually strengthen the mix... up to 3 parts. The mix of choice (concrete) is 1pt cement :2pt sand : 3pt gravel, because it's the best/hardest (for the money).