jmc

We claim that a number has an odd number of positive factors if and only if it is a perfect square. Indeed, for all non-square numbers $x$, we can pair each factor $f$ with another factor $\frac{x}{f}$, so there must be an even number of factors. For perfect squares, this argument fails only for $\sqrt{x}$, so there are an odd number of factors for perfect squares. Thus we seek the greatest perfect square below $100$, which is $\boxed{81}$.