What is the least general category of classification of living things?
The most specific classification one can use is that of species, but sometimes organisms can be classified even further.
The most general classification one can give is the domain of life the organism is in. The most specific level one can describe an organism to is typically thought of as the species level, but sometimes scientists go even further than this.
We can separate species even further to the subspecies level. For example, the Siberian tiger ( Anthera tigris altaica ) is a different subspecies than the Sumatran tiger (Panthera tigris sumatrae).