Which ion is least polarising?

Polarising power is set by charge density, the charge divided by the ionic radius. Using six-coordinate radii: $Al^{3+}$ is 3/54, about 0.056 charge units per pm; $Mg^{2+}$ is 2/72, about 0.028; $Ba^{2+}$ is 2/135, about 0.015; and $Na^+$ is 1/102, about 0.0098. $Na^+$ carries a single charge spread over a large radius, so it has the lowest charge density of the four and is the least polarising. $Ba^{2+}$ is the least polarising of the doubly charged ions, but it still has about one and a half times the charge density of $Na^+$.