The common answer is that Rare Candy is only for legendaries, with Mewtwo, Rayquaza, Kyogre and Groudon most deserving.
However, when I asked whether I should power up my perfect Groudon and perfect Mewtwo, the responses implied that they were not that useful.
If given Overheat, Heatran would rank up there with them.
For competitive gym scenes, Raikou and Chansey are also very worth spending Rare Candy on.