Due to the Isoperimetric inequality the sphere has the minimal surface area compared to ANY other solid. With a greater surface area, the drink probably will cool faster with ice cubes, thus the drink will reach the "steady state" where ice melts slower faster. I would guess that once at the steady state, the rate of melting will be the same for any shape ice. But by the time it's the steady state, won't the cube turn into spheres anyways? Maybe I would rephrase the conditions of the question to adding spheres or cubes to an already cooled drink... But then the answer does not have many practical applications. The easy solution is to buy whiskey stones :p I'm no engineer and i'm pretty curious if anybody can provide a mathematical answer to this question :)