Someone can not say Heat Map is a least effective type of visualization. I would rather say it depends on your requirement. In some case Heat maps are very useful like, let say you have to make a report for crime in a country state wise (or city wise). Here you will have huge data set which can have time dependencies.

Similarly let say you have to prepare a report for electricity consumption for cities. In these cases you can easily visualize through Heat map. It will make more seance and less cumbersome.

So, in nutshell if you have lots of continues data and you want to make a report which can pin point the answers quickly then Heat map is best.

Someone can not say Heat Map is the least effective type of visualization. I would rather say it depends on your requirement. In some cases Heat maps are very useful. Let's say you have to make a report on crime in a country state-wise (or city-wise). Here you will have a huge data set which can have time dependencies.

Similarly, let's say you have to prepare a report on electricity consumption for cities. In these cases you can easily visualize through Heat map. It will make more sense and be less cumbersome.

So, in a nutshell, if you have lots of continuous data and you want to make a report which can pin-point the answers quickly then Heat map is best.