Of course they need have criteria in the first place before creating an index like this. It's better than just randomly saying "I think this country is free, but I have no evidence so I'm going to rely on popular superstition". Now, you can question their criteria, like in your Signapore example, but I think an index like this is better than nothing. Tbh, I feel they did a god job; the criteria is clear and based on research.
And yet they could still be right. That's the problem with an ad hominem attack, it proves nothing about the arguments or information being given by the person or group in question.The publisher of the study (The Heritage Foundation) is an extreme right wing American think tank funded by the Koch brothers; it has a pretty biased agenda.
I personally don't agree with everything they support either, in terms of policy, but I don't see a reason why they would publish an index like this with bias either. And like I said, even if it is published with bias, that isn't the point. Pretty much any public policy think tank is going to be bias as bias in inherent in policy making. theHF is no exception there, that doesn't make their work worthless imo.